Notify users of connection loss so they can re-login

This is expected to fix <https://trello.com/c/ex5Fw8wp/186-1-as-a-user-when-my-connection-to-the-presence-server-is-lost-talkilla-shows-that-i-am-logged-out>, because it handles WebSocket error events, which were previously being noticed but not actually responded to.

Since we don't know what actually triggers the WebSocket error events, it's not clear how one could functionally test this, so I haven't.