[JUJU-759] Change error types to use new ConstError

error.go

@@ -6,7 +6,6 @@ package errors
 import (
 	"fmt"
 	"reflect"
-	"runtime"
 )
 
 // Err holds a description of an error along with information about
@@ -33,6 +32,43 @@ type Err struct {
 	line int
 }
 
+// Locationer is an interface that represents a certain class of errors that
+// contain the location information from where they were raised.
+type Locationer interface {
+	// Location returns the path-qualified function name where the error was
+	// created and the line number
+	Location() (function string, line int)
+}
+
+// locationError is the internal implementation of the Locationer interface.
+type locationError struct {
+	error
+
+	// function is the package path-qualified function name where the
+	// error was created.
+	function string
+
+	// line is the line number the error was created on inside of function
+	line int
+}
+
+// newLocationError constructs a new Locationer error from the supplied error
+// with the location set to callDepth in the stack.
+func newLocationError(err error, callDepth int) *locationError {
+	le := &locationError{error: err}
+	le.function, le.line = getLocation(callDepth + 1)
+	return le
+}
+
+// *locationError implements Locationer.Location interface
+func (l *locationError) Location() (string, int) {
+	return l.function, l.line
+}
+
+func (l *locationError) Unwrap() error {
+	return l.error
+}
+
 // NewErr is used to return an Err for the purpose of embedding in other
 // structures.  The location is not specified, and needs to be set with a call
 // to SetLocation.
@@ -160,14 +196,7 @@ func (unformatter) Format() { /* break the fmt.Formatter interface */ }
 // SetLocation records the package path-qualified function name of the error at
 // callDepth stack frames above the call.
 func (e *Err) SetLocation(callDepth int) {
-	rpc := make([]uintptr, 1)
-	n := runtime.Callers(callDepth+2, rpc[:])
-	if n < 1 {
-		return
-	}
-	frame, _ := runtime.CallersFrames(rpc).Next()
-	e.function = frame.Function
-	e.line = frame.Line
+	e.function, e.line = getLocation(callDepth + 1)
 }
 
 // StackTrace returns one string for each location recorded in the stack of