Comment 1 for bug 1821594

IRC discussion here:

http://eavesdrop.openstack.org/irclogs/%23openstack-nova/%23openstack-nova.2019-03-25.log.html#t2019-03-25T13:07:57

Setting the migration record status to error is easy, so I'll start with that. Refactoring the code to handle errors but still cleanup allocations is a bit more complicated, so that can be split into a separate change to close the bug.

Also note related bug 1793569 with some scripts for cleaning up orphaned allocations.