pgsql-server: Tweak elog.c's logic for promoting errors into more severe

Log Message:
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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.

Modified Files:
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    pgsql-server/src/backend/utils/error:
        elog.c (r1.148 -> r1.149)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/error/elog.c.diff?r1=1.148&r2=1.149)