Correctly handle failed migrations (#17575)

Correctly handle failed migrations (#17575)

* Correctly handle failed migrations

There is a bug in handling failed migrations whereby the migration task gets decoupled
from the migration repository. This leads to a failure of the task to get deleted with
the repository and also leads to the migration failed page resulting in a ISE.

This PR removes the zeroing out of the task id from the migration but also makes
the migration handler tolerate missing tasks much nicer.

Fix #17571

Signed-off-by: Andrew Thornton <art27@cantab.net>