pgsql: Make error message more verbose, in hopes of avoiding

Log Message:
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Make error message more verbose, in hopes of avoiding misunderstandings
such as bug #1293.

Modified Files:
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pgsql/src/backend/parser:
parse_target.c (r1.126 -> r1.127)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_target.c.diff?r1=1.126&r2=1.127)