When an InstructorTask is stuck in QUEUING (say if there was a
problem with celery), the support team needs to manually intervene
and mark the task as "FAILED" so that new tasks of that type can
be created for that course. This is usually done one at a time,
but sometimes a bug or outage might cause many tasks to fail at
once, making recovery extremely cumbersome. This commit adds the
ability to do this process in batches.
