Propagate exceptions thrown from the event handlers

[ffeytons]

It can happen that an error occurs in an event handler e.g. while adding files to the manifest in a listerner of the BackupManifestWasCreated event.
In such a case, the current behaviour is that laravel-backup will still continue its processing and eventually consider the backup as successful, preventing the people monitoring the notifications from knowing about the error.

It might be interesting to consider exceptions sent in event listeners as deal-breakers and to halt the process and trigger the BackupHasFailed event as a consequence.

To illustrate the case, I'm using the BackupManifestWasCreated event to download files to be backed up from an S3 bucket (I want to backup this bucket), store them locally and include them in the backup.
The code I'm using right now to be notified of an issue with my S3 sync implementation is the following:

if (!$s3SyncCommandSuccess) {
  $exception = new Exception("The command 'aws s3 sync' ended with code '" . $s3SyncCommandReturnCode . "' (" . $s3SyncCommandOutput . ")");

  // Hack to force laravel-backup to notify of an error
  event(new BackupHasFailed($exception));

  throw $exception;
}

The issue with this code today is that the backup process will continue anyway (no way to halt it) and I'll receive both a "backup failed" (sent by my code) and a "backup successful" (sent by laravel-backup) notifications.

[PaolaRuby]

Feel free to make a PR for that

[ffeytons]

Done with PR #1559