Implement a proper TUI Error Log.

Implement a proper TUI Error Log.

At last! Now, most errors received from the server will go into a log
that doesn't interrupt and terminate the TUI, and you can at least try
to recover from them and do something else instead.

There's undoubtedly still work to be done in making the errors more
useful, but this is a start. I've left some thoughts in TODO.md.