Fixed deleting instance booted from invalid vol

1. Translated the exception (NotFound) raised from Cinder client to
Nova's native exception (VolumeNotFound).
2. Added unit test coverage.

Fixes LP: #1087214.

Change-Id: I3cee31a9fd068b65a4097f3f3054aebb69d9ba61
(cherry picked from commit 921eec91abee56ea69e1b8b2bff59379b68bff87)