Add exception `MethodNotAllowedError` and handler for it

[redlovv]

Instead of a direct answer from the b"Method not allowed", I propose to process the resulting exception, like other exceptions. Added a MethodNotAllowedError error, as well as a handler for this error.

[duducp]

It would be nice in your pull request to have a description of the changes that have taken place.

[dan83g]

This feature is required. I am waiting too.