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

R=r
CC=golang-dev
https://codereview.appspot.com/13618043
diff --git a/proto/text_test.go b/proto/text_test.go
index f5d0574..c64b073 100644
--- a/proto/text_test.go
+++ b/proto/text_test.go
@@ -303,7 +303,7 @@
 	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()