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.