Tweak elog.c's logic for promoting errors into more severe errors.

Tweak elog.c's logic for promoting errors into more severe errors.
Messages of less than ERROR severity should never be promoted (this
fixes Gaetano Mendola's problem with a COMMERROR becoming a PANIC,
and is obvious in hindsight anyway).  Do all promotion in errstart
not errfinish, to ensure that output decisions are made correctly;
the former coding could suppress logging of promoted errors, which
doesn't seem like a good idea.  Eliminate some redundant code too.