Return correct unknown path/method response

[babolivier]

If a method/path pair is unknown, aiohttp simply sends a response which body is a text representation of the error (e.g. 405: Method Not Allowed). It does so by falling back onto default handlers that raise an HTTPException. We can write a very simple middleware that catches those and turns them into response with Matrix-compatible error bodies.

[DMRobertson]

PC notes in https://github.com/vector-im/matrix-content-scanner-python/pull/49/files#r1406110984 that we should return M_UNRECOGNISED if a (method, pair) path is unknown.

[clokep]

I think the way to do this is to add a middleware which converts HTTPNotFound and HttpMethodNotAllowed errors to the proper JSON responses.

[clokep]

Was kind of easy once I figured out aiohttp, see #55.