code: function onErr(err) {
    // local port is in use
    if (err.code === 'EADDRINUSE') {
      panic('Local port is in use \nFailing...');
    }
    // usbmux not there
    if (err.code === 'ECONNREFUSED' || err.code === 'EADDRNOTAVAIL') {
      panic('Usbmuxd not found at', usbmux.address, '\nFailing...');
    }
    // other
    panic('%s \nFailing...', err);
  }
docs: Error handler for listener and relay
  @param  {Error} err