* 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>
bab95c3a86