pgsql: Make error messages for bad --set-version argument more useful.

Log Message:
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Make error messages for bad --set-version argument more useful.
Per Stefan.

Modified Files:
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pgsql/src/backend/catalog:
genbki.pl (r1.5 -> r1.6)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/genbki.pl?r1=1.5&r2=1.6)