This extends the changes of commit 6252c4f9
so that we run the cleanup hook earlier for failure cases as well as
success cases.  As before, the point is to avoid an assertion failure from
an Assert I added in commit a874fe7b, which
was meant to check that no user-written code can be called during portal
cleanup.  This fixes a case reported by Pavan Deolasee in which the Assert
could be triggered during backend exit (see the new regression test case),
and also prevents the possibility that the cleanup hook is run after
portions of the portal's state have already been recycled.  That doesn't
really matter in current usage, but it foreseeably could matter in the
future.

Back-patch to 9.1 where the Assert in question was added.