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?
Here's the idea: under the `Rocket` namespace should live things critical to
writing simple Rocket apps: Request, Response, Error, etc. Nothing should be
nested more than one level deep. Only items required for more complex things
(implementing uncommon traits, etc.) should be nested one level deep.
This commit is the first attempt at realizing this.
Sergio Benitez