Add ValidURLRouter to handle unmatched routes (#2147)

[sprezz-arthur]

Follow-up from #2147.

My attempt to solve the status code 500 on unmatched routes.
Solving it for http requests was quite straight-forward, as I could return 404 directly, but for websocket requests I could only manage to get a 403 by prematurely closing the connection before the handshake.

To make it non-breaking, I opted for extending URLRouter into ValidURLRouter, as some of the tests in tests/test_routing.py rely on having Exceptions raised.

[carltongibson]

To make it non-breaking, I opted for extending URLRouter into ValidURLRouter, as some of the tests in tests/test_routing.py rely on having Exceptions raised.

You don't seem to have pushed that version 🤔

[sprezz-arthur]

To make it non-breaking, I opted for extending URLRouter into ValidURLRouter, as some of the tests in tests/test_routing.py rely on having Exceptions raised.

You don't seem to have pushed that version 🤔

My bad, bad rebase, should be fine now.

[sprezz-arthur]

Could we give it another go? 😅