code: function onCompletion(err) {
    if (err) {
      // As notifications are tangential to our purpose, just log the error.
      const message =
        err.code === 'ENOENT' ? 'prerequisite software not found' : err.message;
      console.error('notification error:', message);
    }
  }
docs: @summary
  Callback for result of attempted Growl notification.
  
  @description
  Despite its appearance, this is <strong>not</strong> an Error-first
  callback -- all parameters are populated regardless of success.
  
  @private
  @callback Growl~growlCB
  @param {*} err - Error object, or <code>null</code> if successful.