Generalize caught exceptions on workflow delete
Generalize caught exceptions on workflow delete

OsloDB sometimes does not convert the underlying SQLAlchemy
errors into DBError. Sometimes it happens that the "too deep
cascade delete" error manifests in sa.OperationalError instead
of DBError. We generalize to catch both cases as the underlying
issue is the same (just as the message of the error).

Change-Id: Icce8dfa6edb18973ca2a4eb594818ea9d504b49a

openstack/mistral,openstack/mistral