Fixes panics found in Juju for nil errors.

error.go

@@ -53,13 +53,22 @@ type locationError struct {
 }
 
 // newLocationError constructs a new Locationer error from the supplied error
-// with the location set to callDepth in the stack.
+// with the location set to callDepth in the stack. If a nill error is provided
+// to this function then a new empty error is constructed.
 func newLocationError(err error, callDepth int) *locationError {
 	le := &locationError{error: err}
 	le.function, le.line = getLocation(callDepth + 1)
 	return le
 }
 
+// Error implementes the error interface.
+func (l *locationError) Error() string {
+	if l.error == nil {
+		return ""
+	}
+	return l.error.Error()
+}
+
 // *locationError implements Locationer.Location interface
 func (l *locationError) Location() (string, int) {
 	return l.function, l.line

errortypes.go

@@ -85,12 +85,12 @@ func (e *errWithType) Unwrap() error {
 }
 
 func wrapErrorWithMsg(err error, msg string) error {
-	if msg == "" {
-		return err
-	}
 	if err == nil {
 		return stderror.New(msg)
 	}
+	if msg == "" {
+		return err
+	}
 	return fmt.Errorf("%s: %w", msg, err)
 }
 

errortypes_test.go

@@ -167,3 +167,13 @@ func (*errorTypeSuite) TestAllErrors(c *gc.C) {
 
 	runErrorTests(c, errorTests, true)
 }
+
+// TestThatYouAlwaysGetError is a regression test for checking that the wrap
+// constructor for our error types always returns a valid error object even if
+// don't feed the construct with an instantiated error or a non empty string.
+func (*errorTypeSuite) TestThatYouAlwaysGetError(c *gc.C) {
+	for _, errType := range allErrors {
+		err := errType.wrapConstructor(nil, "")
+		c.Assert(err.Error(), gc.Equals, "")
+	}
+}