dgram: make .send cb act as an "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 joyent/node#4846.
Hm, yes -- the lookup for `dne.example.com` is expected to fail. Would adding `dns.setServers([]);` before the lookup accomplish that reliably?