Improve error message when repack against non-existing table.

Previously, pg_repack shows "ERROR: ERROR: relation foo does not
exist" when specify non-existing table. Though the first ERROR
is from pg_repack and the second ERROR is from PostgreSQL server,
some users might think that pg_repack shows error level twice
wrongly.

bin/pg_repack.c

@@ -341,7 +341,7 @@ main(int argc, char *argv[])
 		{
 			if (!repack_one_database(orderby, errbuf, sizeof(errbuf)))
 				ereport(ERROR,
-					(errcode(ERROR), errmsg("%s", errbuf)));
+					(errcode(ERROR), errmsg("pg_repack fails by PostgreSQL server error: %s", errbuf)));
 		}
 	}