Middleware to show better 4xx, 5xx responses

[potomak]

[knutin]

I'm a bit reluctant to a catch-all in the middleware, as this makes it tricky to call the handle_event/3 callbacks for request_{exception,error,exit}. An option would be to call it from the middleware explicitly.

I will have to think about it for a bit.. :-)

[potomak]

While you were thinking I've tried to solve the problem in the easiest way :grin:

Notes:

• this pull request solves the problem of missing postprocess call for error requests (was it a problem to solve?)
• the way I've tested handle_event/3 callback is primitive, I'd really like to see how you would achieve this task
• this solution highlights problems of handling middlewares with a plain elli handler, it's elegant, but there should be a better way to do it