The error function now takes in a "RoutingError" structure. The idea is that the
structure includes all of the information necessary for a user to processor the
error as they wish. This interface is very incomplete and may change. At a
minimum, the error structure should include:
1) The request that failed.
2) Why the request failed.
3) The chain of attempted route matches, if any.
4) Something else?
A few important things needs to get this to be 'right':
1a. Have a way to return a response with a status code.
1b. Use that mechanism in the default catchers.
2. Automatically fill in that code from the #[error] handler.
3. Have a way for a responder to say if responding succeeded.
4. Try next highest ranking route if responding with one handler fails.
Added `error` decorator and `errors` macro.
The current idea is that you can have "catchers" for all valid errors code (in
range [400, 500). At the moment, catchers are just request handlers, and the
decorator expected an empty function signature for the error handler. Obviously,
this is pretty useless. Not sure on what the API should be here. But, progress.
Oh, one more thing: who should handle forwarding a request to a catcher?
Probably not the router. So, the main Rocket should?
