Catch NotFound for every stack delete db operation

A stack delete can be interrupted by another delete request
at any stage, so a stack may be in any state of partial deletion.

This change ensures that every db operation in a stack delete
will catch NotFound exceptions so that the delete process can
continue.

Change-Id: I3b665acbc7979147f24793a314da37150d4efeb8
Partial-Bug: 1328983
Merge "Catch NotFound for every stack delete db operation"