goprotobuf: Consistently prefix error strings with "proto: ".

R=r
CC=golang-dev
https://codereview.appspot.com/13618043

proto/decode.go

@@ -46,7 +46,7 @@ import (
 // ErrWrongType occurs when the wire encoding for the field disagrees with
 // that specified in the type being decoded.  This is usually caused by attempting
 // to convert an encoded protocol buffer into a struct of the wrong type.
-var ErrWrongType = errors.New("field/encoding mismatch: wrong type for field")
+var ErrWrongType = errors.New("proto: field/encoding mismatch: wrong type for field")
 
 // errOverflow is returned when an integer is too large to be represented.
 var errOverflow = errors.New("proto: integer overflow")

proto/extensions.go

@@ -109,11 +109,11 @@ func isExtensionField(pb extendableProto, field int32) bool {
 func checkExtensionTypes(pb extendableProto, extension *ExtensionDesc) error {
 	// Check the extended type.
 	if a, b := reflect.TypeOf(pb), reflect.TypeOf(extension.ExtendedType); a != b {
-		return errors.New("bad extended type; " + b.String() + " does not extend " + a.String())
+		return errors.New("proto: bad extended type; " + b.String() + " does not extend " + a.String())
 	}
 	// Check the range.
 	if !isExtensionField(pb, extension.Field) {
-		return errors.New("bad extension number; not in declared ranges")
+		return errors.New("proto: bad extension number; not in declared ranges")
 	}
 	return nil
 }
@@ -272,7 +272,7 @@ func decodeExtension(b []byte, extension *ExtensionDesc) (interface{}, error) {
 func GetExtensions(pb Message, es []*ExtensionDesc) (extensions []interface{}, err error) {
 	epb, ok := pb.(extendableProto)
 	if !ok {
-		err = errors.New("not an extendable proto")
+		err = errors.New("proto: not an extendable proto")
 		return
 	}
 	extensions = make([]interface{}, len(es))
@@ -292,7 +292,7 @@ func SetExtension(pb extendableProto, extension *ExtensionDesc, value interface{
 	}
 	typ := reflect.TypeOf(extension.ExtensionType)
 	if typ != reflect.TypeOf(value) {
-		return errors.New("bad extension value type")
+		return errors.New("proto: bad extension value type")
 	}
 
 	pb.ExtensionMap()[extension.Field] = Extension{desc: extension, value: value}

proto/text_parser.go

@@ -193,8 +193,8 @@ func (p *textParser) advance() {
 }
 
 var (
-	errBadUTF8 = errors.New("bad UTF-8")
-	errBadHex  = errors.New("bad hexadecimal")
+	errBadUTF8 = errors.New("proto: bad UTF-8")
+	errBadHex  = errors.New("proto: bad hexadecimal")
 )
 
 func unquoteC(s string, quote rune) (string, error) {

proto/text_test.go

@@ -303,7 +303,7 @@ type limitedWriter struct {
 	limit int
 }
 
-var outOfSpace = errors.New("insufficient space")
+var outOfSpace = errors.New("proto: insufficient space")
 
 func (w *limitedWriter) Write(p []byte) (n int, err error) {
 	var avail = w.limit - w.b.Len()