fix(gcr/c) print a loud error when clients receive routing messages, but don't panic

This is an unhandled case. Right now, we close the stream without
reading. Should clients be able to satisfy routing requests?

@jbenet @whyrusleeping

routing/grandcentral/proxy/standard.go

@@ -33,7 +33,9 @@ func Standard(h host.Host, remotes []peer.ID) Proxy {
 }
 
 func (p *standard) HandleStream(s inet.Stream) {
-	panic("client received a GCR message")
+	// TODO(brian): Should clients be able to satisfy requests?
+	log.Error("grandcentral client received (dropped) a routing message from", s.Conn().RemotePeer())
+	s.Close()
 }
 
 func (px *standard) SendMessage(ctx context.Context, m *dhtpb.Message) error {