print error's string when panicking.

R=rsc
https://golang.org/cl/157156

src/pkg/rpc/client.go

@@ -61,7 +61,7 @@ func (client *Client) send(c *Call) {
 	client.enc.Encode(request);
 	err := client.enc.Encode(c.Args);
 	if err != nil {
-		panicln("rpc: client encode error:", err)
+		panicln("rpc: client encode error:", err.String())
 	}
 	client.sending.Unlock();
 }