pgsql: Improve error wording of ORDER BY in UNION that uses new

Log Message:
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Improve error wording of ORDER BY in UNION that uses new expressions in
ORDER BY.

Modified Files:
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pgsql/src/backend/parser:
analyze.c (r1.355 -> r1.356)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c.diff?r1=1.355&r2=1.356)