This event will be fired when something goes wrong while backing up.
It has two public properties:
$exception: an object that extends PHP's Exception-class. It is highly likely that $exception->getMessage() will return more information on what went wrong.
$backupDestination: if this is null then probably something went wrong zipping the files to be backed up. If it's an instance of Spatie\Backup\BackupDestination\BackupDestination something went wrong copying the zip over to that destination.