Allow delete when InstanceInfoCache entry is missing

The existing code wouldn't allow you to delete 'broken' instances that were
(for whatever reason) missing their InstanceInfoCache entry. This patch fixes
it such that those instances can be deleted.

Adds a test case to cover the new exception handling.

Closes-Bug: #1266919

Co-Authored-By: Matt Riedemann <mriedem@us.ibm.com>
Change-Id: I3d566a25e340b5d5de86740141e1eaa5b3742915
Merge "Allow delete when InstanceInfoCache entry is missing"