Return JSON error responses for unknown routes/methods.

[clokep]

I tested this with:

$ curl -v http://localhost:8080/_matrix/client/foo
$ curl -v http://localhost:8080/_matrix/media_proxy/unstable/foo
$ curl -v -X DELETE http://localhost:8080/_matrix/media_proxy/unstable/download/foo/bar

Fixes #43

[DMRobertson]

Seems fine to me. Would be nice to write up your curl testing as automated tests, but I won't block this on that.

[clokep]

Seems fine to me. Would be nice to write up your curl testing as automated tests, but I won't block this on that.

I couldn't see anything that tests things at the REST layer, unfortunate, so I'm not really sure how to do that.