3PAR: Delete missing snapshot stuck in error_del

If the snapshot is deleted outside of cinder, attempting
to delete it again results in the snapshot state stuck in
error_deleting.

This is because the current code raises a NotFound exception,
but the base cinder code does not catch this and update its db.

Instead, log a warning, and simply tell the base cinder code that it was
successfully deleted.

Change-Id: I5c5d24bc113e320346ff4f1f4f62fffdf7cddfc4
Closes-Bug: #1283233
Merge "3PAR: Delete missing snapshot stuck in error_del"