Emit middlewareError when on middleware error.

Created by: philjackson

A couple of patches (one updating the README) which allow a user to handle middleware errors in much the same way they might in express. For example, within the middleware calling:

next(new Error("I'm a gonner"));

Now emits middlewareError and can be handled later like this:

 server.proxy.on('middlewareError', function (err, res, req) {
   // handle the error here and call res.end()
 });

This change is backwardly compatible and the old behaviour persists if nothing heard the middlewareError event.