dgram: report send errors to cb, don't pass bytes

Passing the number of sent bytes to the callback is superfluous;
datagram sockets operate in atomic mode: either the sendmsg() system
call succeeds or it fails but it never does partial writes.

Instead, report send errors to the callback. UDP error reporting is
fairly haphazard on most platforms. You should not expect reliable
delivery of anything besides EMSGSIZE and (possibly) ENETDOWN and
ENETUNREACH.

Fixes #2608.