Add "421 Misdirected Request" http exception

[n1ngu]

Related: #1256

Any concerns against adding a http exception class for this status code?

Right now aborting fails with a LookupError: no exception for <HTTPStatus.MISDIRECTED_REQUEST: 421> error if you try

abort(http.HTTPStatus.MISDIRECTED_REQUEST)

The Aborter class can be enriched to feature extra status codes and the Response class can be used to return arbitrary status codes and messages. But wouldn't it make sense that this http exception was featured out of the box?