This commit modifies `codegen` so that a route's name (the name of the
route handler) is stored in the generated static route information
structure and later propogated into the corresponding `Route`
structure.
The primary advantage of this change is an improvement to debug and
error messages which now include route names. The collision error
message, in particular, has been improved dramatically in this commit.
Additionally, the `LaunchError::Collision` variant now contains a
vector of the colliding routes.
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?
