1128061: Don't raise logged Disconnected on unreg

BaseActionInvoker handles and logs any exceptions
raise from *certlib actions. For unregister, when we
run the UnregisterAction, we have already deleted
id cert, ent certs, repos, so we would always raise
Disconnected, and then it would get logged.

In either cause, entcertlib's actions don't do much,
so just return empty results instead of raising an
exception that gets logged.
Merge pull request #1019 from candlepin/alikins/1128061_no_disconnected

1128061: Don't raise logged Disconnected on unreg