Handle instance being deleted while in filter scheduler

Adds handling of InstanceNotFound when trying to update the
scheduled_at time of an instance.

Users can delete an instance at any time (and many test
cases delete very quickly), so this should be a warning
not an error

Fixes bug 1191126

Change-Id: I72b296668f60b1de71ff8c6d7bbb749023d89f12
Merge "Handle instance being deleted while in filter scheduler"