Split off from MDL-68773

This leads to the adhoc task failing due to being unable to find the target course.

Findings from there:

The main issue I found is that although there is protection on deleting courses on the UI delete page (/course/delete.php), there are still a number of ways to delete the course before the backup is completed.

It may be better to move the check that looks for a pending backup test into the core api function `delete_courses`. The current implementation - https://github.com/moodle/moodle/blob/master/course/delete.php#L85

Replication Steps

A. core\task\course_backup_task

1. In admin settings, enable asynchronous backups.
2. Create a course.
3. Go to course page.
4. Create a backup of course.
5. Check that the backup is in the pending state.
6. Call the external function `core_course_delete_courses` with the id of the course that has a pending backup.
7. Run the cron tasks.
8. Check that the error appears as the course to backup no longer exists.