BB-4198: Fix issues with package upgrades.

Fixes two issues:

1. From Openstack package upgrade: an exception class was removed in favor of a more generic one.
2. Bug found while testing: Terminate VM is not a background task, which makes the web worker halt if the VM takes too long to shutdown (moved to a huey task).

This is currently deployed to stage.

Reviewers:

• Any FFs: @Clemente @symbolist @itsjeyd (high priority review, some Ocim methods are broken on master without this)