Implement default handler if nothing matches

There's an undocumented(?) event handler named `all_events` but I'd also like to have a catch-all for unhandled events called `unhandled_event` or something so handlers can be registered at reactor level directly where they exist and others fall through to a catch-all.

I'm now at a point I either need to refactor my project completely to use a custom event handler based on `all_events` or get an upstream solution as there's too many non-standard reply codes to catch all of them manually.

Would this be doable?

Thanks.

Start by reading the existing `all_events` handler and the reactor-level event registration path. Clarify the intended name and dispatch behavior for a catch-all handler, including how unhandled events and non-standard reply codes are identified. Done should include an agreed upstream API and coverage for the fallback behavior.

Written by the indexing model from the issue text.