Clarify pg_upgrade error message that the 'postgres' database must exist

in the old cluster.

contrib/pg_upgrade/check.c

@@ -403,9 +403,14 @@ check_old_cluster_has_new_cluster_dbs(void)
 					   new_cluster.dbarr.dbs[new_dbnum].db_name) == 0)
 				break;
 		if (old_dbnum == old_cluster.dbarr.ndbs)
+		{
+			if (strcmp(new_cluster.dbarr.dbs[new_dbnum].db_name, "postgres") == 0)
+				pg_log(PG_FATAL, "The \"postgres\" database must exist in the old cluster\n");
+			else
 				pg_log(PG_FATAL, "New cluster database \"%s\" does not exist in the old cluster\n",
 					   new_cluster.dbarr.dbs[new_dbnum].db_name);
 		}
+	}
 }