Re: error_severity of brin work item

The more I look at this, the less I like it. This would set a precedent
that any action that can be initiated from an autovac work-item has a
requirement of silently being discarded when it referenced a
non-existant relation.

I'd rather have the code that drops the index go through the list of
work-items and delete those that reference that relation.

I'm not sure if this is something that ought to be done in index_drop();
One objection might be that if the drop is rolled back, the work-items
are lost. It's the easiest, though; and work-items are supposed to be
lossy anyway, and vacuum would fix the lack of summarization eventually.
So, not pretty, but not all that bad. (Hopefully rolled-back drops are
not all that common.)