Add ERROR as possible job status

If a job fails, its status is set to `'ERROR'` (see [line 171](https://github.com/mlocati/concrete5/blob/8970aea96ca209c1f9e0622c582614715cf50c42/web/concrete/core/models/job.php#L171)). BTW that state is not included in the list of available statuses (`$availableJStatus`)