Refactor UnexpectedTaskStateError for handling of deleting instances

To avoid having to repeatedly unfold UnexpectedTaskStateErrors and
do seperate handling of the 'deleting' task state from other states,
everywhere this exception is handled, introduce a separate exception
UnexpectedDeletingTaskStateError that derives from the above.

This way, most handling of InstanceNotFound to handle instances gone
during some action can be made to also include this new exception.

Change-Id: Ia833c2b2361f32c7e33c84ec14d3af6231119a99
Related-Bug: #1223843
Merge "Refactor UnexpectedTaskStateError for handling of deleting instances"