elog levels for _redo failures

I notice that there is some variation in the way that different rmgrs
use elog levels.

Heap uses PANIC always
BTree uses LOG and PANIC
GIN uses ERROR always
GIST uses ERROR always

Btree issues a LOG during forget_matching_split(), but does nothing
during forget_matching_deletion(). 

GIN and GIST now don't issue any elog at all when they cannot find a
matching split. GIN doesn't complain if it cannot cleanup correctly,
while GIST does.

Is there a particular reason or benefit for this much variation in the
code paths for each rmgr? Why do the log levels vary?

I'm thinking that index rmgr log levels should be either LOG or PANIC,
in this release, plus I would expect to see some more LOGs for when
things don't work right. Or is there no need to PANIC? ;-)

--  Simon Riggs 2ndQuadrant  http://www.2ndQuadrant.com