Added proxy error handling middleware support.

[olysyuk]

It would be nice to handle proxy errors (show nice page on 404, or pass call to another middleware if proxy fails).

In my case I want to set up connect server, that will route request between frontend grunt serve and backend grunt serve.

Then I can just set up:

1. proxy to frontend,
2. if 404, than it's not a frontend file and backend proxy should handle the request.