Show a client error if the WebSocket connection closes

Instead of automatically reconnecting when the WebSocket connection dies for whatever reason, we show an error asking that the user refresh.

This PR also DRYs out our existing error handling a bit.
Ah, I see. Is there any reason we can't put it in `community.core/init-app` instead? `window.onload` waits until all resources are loaded (including pictures, etc.) which on long threads with lots of posts may take a bit of time.