Catch zmq socket_t::bind() failures on startup

Without this zmq socket_t::bind() failure is printed on stdout (and
difficult to track when started in background and with initscript)

Feel free to manage differently, just want to have the error message in logs in case of startup failure (EADDRINUSE).
for the moment we do not use that, but wouldn't that be better to return an error code != 0 ?