Always wrap user errors

This is an alternative to #1305 that leans into error wrapping instead of away from it.

Requires use of go 1.13 error unwrapping.

On measure I think I prefer this approach, even though it's a bigger BC break:
- There's less mutex juggling
- It has never felt right to me that we make the user deal with path when overriding the error presenter
- The default error presenter is now incredibly simple

Questions:
- Are we comfortable with supporting 1.13 and up?
- Should we change the signature of `ErrorPresenterFunc` to `func(ctx context.Context, err *gqlerror.Error) *gqlerror.Error`?
    - It always is now, and breaking BC will force users to address the requirement for `errors.As`

codegen/args.gotpl

@@ -5,13 +5,13 @@ func (ec *executionContext) {{ $name }}(ctx context.Context, rawArgs map[string]
 	{{- range $i, $arg := . }}
 		var arg{{$i}} {{ $arg.TypeReference.GO | ref}}
 		if tmp, ok := rawArgs[{{$arg.Name|quote}}]; ok {
-			ctx := graphql.WithFieldInputContext(ctx, graphql.NewFieldInputWithField({{$arg.Name|quote}}))
+			ctx := graphql.WithPathContext(ctx, graphql.NewPathWithField({{$arg.Name|quote}}))
 			{{- if $arg.ImplDirectives }}
 				directive0 := func(ctx context.Context) (interface{}, error) { return ec.{{ $arg.TypeReference.UnmarshalFunc }}(ctx, tmp) }
 				{{ template "implDirectives" $arg }}
 				tmp, err = directive{{$arg.ImplDirectives|len}}(ctx)
 				if err != nil {
-					return nil, err
+					return nil, graphql.ErrorOnPath(ctx, err)
 				}
 				if data, ok := tmp.({{ $arg.TypeReference.GO | ref }}) ; ok {
 					arg{{$i}} = data
@@ -20,7 +20,7 @@ func (ec *executionContext) {{ $name }}(ctx context.Context, rawArgs map[string]
 						arg{{$i}} = nil
 				{{- end }}
 				} else {
-					return nil, fmt.Errorf(`unexpected type %T from directive, should be {{ $arg.TypeReference.GO }}`, tmp)
+					return nil, graphql.ErrorOnPath(ctx, fmt.Errorf(`unexpected type %T from directive, should be {{ $arg.TypeReference.GO }}`, tmp))
 				}
 			{{- else }}
 				arg{{$i}}, err = ec.{{ $arg.TypeReference.UnmarshalFunc }}(ctx, tmp)

codegen/directives.gotpl

@@ -141,7 +141,7 @@ func (ec *executionContext) _subscriptionMiddleware(ctx context.Context, obj *as
 		{{- end }}
 		res, err := ec.ResolverMiddleware(ctx, next)
 		if err != nil {
-			ec.Error(ctx, err)
+				ec.Error(ctx, err)
 			return nil
 		}
 		return res

codegen/field.gotpl

@@ -23,7 +23,7 @@ func (ec *executionContext) _{{$object.Name}}_{{$field.Name}}(ctx context.Contex
 		rawArgs := field.ArgumentMap(ec.Variables)
 		args, err := ec.{{ $field.ArgsFunc }}(ctx,rawArgs)
 		if err != nil {
-			ec.Error(ctx, err)
+				ec.Error(ctx, err)
 			return {{ $null }}
 		}
 		fc.Args = args
@@ -81,7 +81,7 @@ func (ec *executionContext) _{{$object.Name}}_{{$field.Name}}(ctx context.Contex
 		{{ template "implDirectives" . }}
 		tmp, err := directive{{.ImplDirectives|len}}(rctx)
 		if err != nil {
-			return nil, err
+			return nil, graphql.ErrorOnPath(ctx, err)
 		}
 		if tmp == nil {
 		    return nil, nil

codegen/input.gotpl

@@ -17,13 +17,13 @@
 			case {{$field.Name|quote}}:
 				var err error
 
-				ctx := graphql.WithFieldInputContext(ctx, graphql.NewFieldInputWithField({{$field.Name|quote}}))
+				ctx := graphql.WithPathContext(ctx, graphql.NewPathWithField({{$field.Name|quote}}))
 				{{- if $field.ImplDirectives }}
 					directive0 := func(ctx context.Context) (interface{}, error) { return ec.{{ $field.TypeReference.UnmarshalFunc }}(ctx, v) }
 					{{ template "implDirectives" $field }}
 					tmp, err := directive{{$field.ImplDirectives|len}}(ctx)
 					if err != nil {
-						return it, err
+						return it, graphql.ErrorOnPath(ctx, err)
 					}
 					if data, ok := tmp.({{ $field.TypeReference.GO | ref }}) ; ok {
 						it.{{$field.GoFieldName}} = data
@@ -32,7 +32,8 @@
 							it.{{$field.GoFieldName}} = nil
 					{{- end }}
 					} else {
-						return it, fmt.Errorf(`unexpected type %T from directive, should be {{ $field.TypeReference.GO }}`, tmp)
+						err := fmt.Errorf(`unexpected type %T from directive, should be {{ $field.TypeReference.GO }}`, tmp)
+						return it, graphql.ErrorOnPath(ctx, err)
 					}
 				{{- else }}
 					it.{{$field.GoFieldName}}, err = ec.{{ $field.TypeReference.UnmarshalFunc }}(ctx, v)

codegen/testserver/directive_test.go

@@ -200,7 +200,7 @@ func TestDirectives(t *testing.T) {
 
 			err := c.Post(`query { directiveArg(arg: "") }`, &resp)
 
-			require.EqualError(t, err, `[{"message":"invalid length","path":["directiveArg"]}]`)
+			require.EqualError(t, err, `[{"message":"invalid length","path":["directiveArg","arg"]}]`)
 			require.Nil(t, resp.DirectiveArg)
 		})
 		t.Run("when function errors on nullable arg directives", func(t *testing.T) {
@@ -210,7 +210,7 @@ func TestDirectives(t *testing.T) {
 
 			err := c.Post(`query { directiveNullableArg(arg: -100) }`, &resp)
 
-			require.EqualError(t, err, `[{"message":"too small","path":["directiveNullableArg"]}]`)
+			require.EqualError(t, err, `[{"message":"too small","path":["directiveNullableArg","arg"]}]`)
 			require.Nil(t, resp.DirectiveNullableArg)
 		})
 		t.Run("when function success on nullable arg directives", func(t *testing.T) {
@@ -317,7 +317,7 @@ func TestDirectives(t *testing.T) {
 
 			err := c.Post(`query { directiveInputNullable(arg: {text:"invalid text",inner:{message:"123"}}) }`, &resp)
 
-			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg"]}]`)
+			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg","text"]}]`)
 			require.Nil(t, resp.DirectiveInputNullable)
 		})
 		t.Run("when function errors on inner directives", func(t *testing.T) {
@@ -327,7 +327,7 @@ func TestDirectives(t *testing.T) {
 
 			err := c.Post(`query { directiveInputNullable(arg: {text:"2",inner:{message:""}}) }`, &resp)
 
-			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg","inner"]}]`)
+			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg","inner","message"]}]`)
 			require.Nil(t, resp.DirectiveInputNullable)
 		})
 		t.Run("when function errors on nullable inner directives", func(t *testing.T) {
@@ -337,7 +337,7 @@ func TestDirectives(t *testing.T) {
 
 			err := c.Post(`query { directiveInputNullable(arg: {text:"success",inner:{message:"1"},innerNullable:{message:""}}) }`, &resp)
 
-			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg","innerNullable"]}]`)
+			require.EqualError(t, err, `[{"message":"not valid","path":["directiveInputNullable","arg","innerNullable","message"]}]`)
 			require.Nil(t, resp.DirectiveInputNullable)
 		})
 		t.Run("when function success", func(t *testing.T) {
@@ -413,7 +413,7 @@ func TestDirectives(t *testing.T) {
 
 				err := c.WebsocketOnce(`subscription { directiveArg(arg: "") }`, &resp)
 
-				require.EqualError(t, err, `[{"message":"invalid length","path":["directiveArg"]}]`)
+				require.EqualError(t, err, `[{"message":"invalid length","path":["directiveArg","arg"]}]`)
 				require.Nil(t, resp.DirectiveArg)
 			})
 			t.Run("when function errors on nullable arg directives", func(t *testing.T) {
@@ -423,7 +423,7 @@ func TestDirectives(t *testing.T) {
 
 				err := c.WebsocketOnce(`subscription { directiveNullableArg(arg: -100) }`, &resp)
 
-				require.EqualError(t, err, `[{"message":"too small","path":["directiveNullableArg"]}]`)
+				require.EqualError(t, err, `[{"message":"too small","path":["directiveNullableArg","arg"]}]`)
 				require.Nil(t, resp.DirectiveNullableArg)
 			})
 			t.Run("when function success on nullable arg directives", func(t *testing.T) {