fix bug causing empty strings to be become non-nil errors on client side of rpc connection.

R=rsc
CC=golang-dev
https://golang.org/cl/155078

src/pkg/rpc/client.go

@@ -83,7 +83,12 @@ func (client *Client) input() {
 		client.pending[seq] = c, false;
 		client.mutex.Unlock();
 		err = client.dec.Decode(c.Reply);
-		c.Error = os.ErrorString(response.Error);
+		// Empty strings should turn into nil os.Errors
+		if response.Error != "" {
+			c.Error = os.ErrorString(response.Error)
+		} else {
+			c.Error = nil
+		}
 		// We don't want to block here.  It is the caller's responsibility to make
 		// sure the channel has enough buffer space. See comment in Go().
 		_ = c.Done <- c;	// do not block

src/pkg/rpc/server.go

@@ -276,7 +276,9 @@ func sendResponse(sending *sync.Mutex, req *Request, reply interface{}, enc *gob
 	resp := new(Response);
 	// Encode the response header
 	resp.ServiceMethod = req.ServiceMethod;
-	resp.Error = errmsg;
+	if errmsg != "" {
+		resp.Error = errmsg
+	}
 	resp.Seq = req.Seq;
 	sending.Lock();
 	enc.Encode(resp);