http: handle errors on idle sockets

This change adds a new event handler to the `error` event of the socket
after it has been used by the http_client.

The purpose of this change is to catch errors on *keep alived*
connections from idle sockets that otherwise will cause an uncaugh error
event on the application.

Fix: #3595
PR-URL: https://github.com/nodejs/node/pull/4482


Reviewed-By: Fedor Indutny <fedor@indutny.com>