Handle UnexpectedTaskState and InstanceNotFound exceptions

UnexpectedTaskState and InstanceNotFound are thrown when a delete
of the instance happens before the ongoing action is completed.
These are to be handled gracefully.
These exceptions have to be logged and not reraised, as they are false
alarms.

Fixes: bug #1186447

Change-Id: I0647935c4452d4294f49a4c0d08e480584ebec7c
Merge "Handle UnexpectedTaskState and InstanceNotFound exceptions"