XEP-0198: Don't exit on socket send failure

If stream management is enabled, don't exit the c2s process when
ejabberd_socket:send/2 fails, but close the socket instead.  This gives
the client a chance to resume the session.

Thanks go to Matthias Rieber for reporting the issue, providing detailed
logs, and testing the fix.
Merge pull request #220 from weiss/handle-send-failure

XEP-0198: Don't exit on socket send failure