Add WebsocketServer exception handler

[davidbrochart]

The ability to auto-restart is only supported when using the start/stop API for now, not the context manager.
It does so by catching exceptions and re-creating a new task group.

[jzhang20133]

When the original task group fail, all its tasks are cancelled, including all the existing rooms's broadcast_update tasks, do we need to find a way to recover them?
Is it possible to have one task group per room so one room throws exception in broadcast_update task won't also tear down other room's broadcast_update task?

[jzhang20133]

Could we also add auto_restart for yroom's task group as well?

[Zsailer]

I think we should explore recovering each individual room too, but maybe let's leave this for a follow-up PR?

At the very minimum we need recovery at the top layer.

[Zsailer]

This looks good to me @davidbrochart.

[davidbrochart]

I removed auto_restart and instead added exception_handler, which is a callback that is called when an exception is raised. The exception handler returns True if the exception was handled, and in this case the exception is not raised, otherwise it is raised. An exception_logger is available, that just logs the exception and discards it. This is the one we want to use if we absolutely don't want the WebSocket server to crash.

There are two places where exceptions can be handled:

• at the top-level where the server is waiting for connections,
• when serving a WebSocket.

The WebSocket server task group is re-created upon successful exception handling, so that it always stays alive.
Each WebSocket connection has its own task group, so that a successful exception handling at this level doesn't affect the top-level task group.

[Zsailer]

I like this a lot, @davidbrochart. Great work!

[Zsailer]

this looks good, @davidbrochart. I propose we merge here and work on getting #30 to follow this pattern. If we notice any changes needed here in that PR, we can fix there.

I'll work with @jzhang20133 to get these two aligned.

[davidbrochart]

Sounds good.