pgsql: Make pg_dump emit a useful error message, instead of just dumping

Log Message:
-----------
Make pg_dump emit a useful error message, instead of just dumping core,
if it finds a pg_rewrite entry for which there is no pg_class entry.
Per report from Andrew Slobodyanyk.

Modified Files:
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pgsql/src/bin/pg_dump:
pg_dump.c (r1.403 -> r1.404)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c.diff?r1=1.403&r2=1.404)