dgram: make send cb act as "error" event handler

This allows users to provide a callback that handles potential
errors resulting from a `socket.send` call. The original behavior
of emitting the error event on the socket is preserved.

Fixes: https://github.com/joyent/node/issues/4846
PR-URL: https://github.com/joyent/node/pull/7738
PR-URL: https://github.com/nodejs/io.js/pull/1796


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>