Middleware to show better 4xx, 5xx responses

[potomak]

I've added exception handling in middleware process execution to postprocess also error responses.

Closes #46

[knutin]

Hi, thanks for the patch!

Your implementation is a bit different than what I had in mind.

In the branch request_id I have added an id to each request that is also available in the request_{throw,error,exit} events. The request id will be sent to the browser, much like what Varnish does. https://github.com/wooga/elli_access_log can use this request id. Or if you have your own error logging middleware that for example logs to a database, you can use the request id as primary key.

What do you think?

[potomak]

I didn't see the request_id branch!
That makes a lot of sense, without the elli_request:id I just used the elli_request:to_proplist as you are using in https://github.com/wooga/elli_access_log/blob/master/src/elli_access_log.erl#L81.

The only tricky part of this pull request is the exception handling in elli_middleware.erl that makes always run the postprocess method post-processing the response.

What do you think?

I'd be happy to update the middleware to follow your specs, is it ok for you?

[potomak]

See #62