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

R=r
CC=golang-dev
https://codereview.appspot.com/13618043
diff --git a/proto/text_parser.go b/proto/text_parser.go
index f39b908..13827f6 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -193,8 +193,8 @@
 }
 
 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) {