Consider passing request to error handlers (404/500)

[m4tx]

Currently, the error handlers are not very nice to use, because the Request object is not passed to them. This is for good reasons (an owned object is passed to the request handler, both for performance and usability reasons), but this limits what the error handlers can do. Particularly, they cannot reverse URLs.

If this proves to be hard or not performant enough, we should at least consider creating a fake Request object that would contain some "globally-available" stuff, like the root router to allow cot::reverse! usage.