goprotobuf: Remove ErrWrongType and improve error messages.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://codereview.appspot.com/110080043
diff --git a/proto/message_set.go b/proto/message_set.go
index a3f5003..6ddcc30 100644
--- a/proto/message_set.go
+++ b/proto/message_set.go
@@ -127,7 +127,7 @@
 
 	mti, ok := pb.(messageTypeIder)
 	if !ok {
-		return ErrWrongType // TODO: custom error?
+		return ErrNoMessageTypeId
 	}
 
 	mtid := mti.MessageTypeId()