`asynchronous_copy_task` adhoc task leaves backup controller record with 800 state

Had a couple of `asynchronous_copy_task` tasks recently that backed up original course but later failed on restore.

During the next run they couldn't load backup controller https://github.com/moodle/moodle/blob/5500d143f406baa5046fd03a00e88e4cfc5f4bb9/lib/classes/task/asynchronous_copy_task.php#L64, completed the adhoc task and left restore backup_controller record with state of 800.

This causes /backup/copyprogress.php?id=123456 page to display copy tasks in progress which will never complete.