Kick Relay clients when backend is offline

[GameParrot]

When using a relay, I get stuck on loading resource packs. I saw #82, which has been marked as fixed a while ago. Seems like the problem came back.
EDIT: The server I was connecting to had crashed, but it should still handle a situation like this better.

[extremeheat]

The Relay is purely intended for debugging. It's not a general proxy solution. However, we can definitely send a kick message on connect timeout, open for PRs on that.