Merge: Nitweb errors

Better error handling for the nitweb API:

* the API catches all unmatched `/api/*` requests and display a JSON error. Example here: http://nitweb.moz-code.org/api/foo
* better error messages (in JSON) for all API handlers. One example here: http://nitweb.moz-code.org/api/entity/foo

Also one change in the frontend that displays 404 not found pages: http://nitweb.moz-code.org/foo

Pull-Request: #2289
Reviewed-by: Jean Privat <jean@pryen.org>