refactor: Use typeMismatchError rather than raw string error (#826)

Uses the existing method to DRY up the error message generation, and decorates
with position index where needed. No behaviour is changed, but it allows for
further changes to make error messaging more specific.

Related to: #806

unmarshaler.go

@@ -339,13 +339,13 @@ func (d *decoder) handleArrayTableCollectionLast(key ast.Iterator, v reflect.Val
 	case reflect.Array:
 		idx := d.arrayIndex(true, v)
 		if idx >= v.Len() {
-			return v, fmt.Errorf("toml: cannot decode array table into %s at position %d", v.Type(), idx)
+			return v, fmt.Errorf("%s at position %d", d.typeMismatchError("array table", v.Type()), idx)
 		}
 		elem := v.Index(idx)
 		_, err := d.handleArrayTable(key, elem)
 		return v, err
 	default:
-		return reflect.Value{}, fmt.Errorf("toml: cannot decode array table into a %s", v.Type())
+		return reflect.Value{}, d.typeMismatchError("array table", v.Type())
 	}
 }
 
@@ -390,7 +390,7 @@ func (d *decoder) handleArrayTableCollection(key ast.Iterator, v reflect.Value)
 	case reflect.Array:
 		idx := d.arrayIndex(false, v)
 		if idx >= v.Len() {
-			return v, fmt.Errorf("toml: cannot decode array table into %s at position %d", v.Type(), idx)
+			return v, fmt.Errorf("%s at position %d", d.typeMismatchError("array table", v.Type()), idx)
 		}
 		elem := v.Index(idx)
 		_, err := d.handleArrayTable(key, elem)