Event error reporting in "knotc -b" and some event handlers updates

src/knot/events/handlers/flush.c

-/*  Copyright (C) 2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -29,10 +29,5 @@ int event_flush(conf_t *conf, zone_t *zone)
 		return KNOT_EOK;
 	}
 
-	int ret = zone_flush_journal(conf, zone, true);
-	if (ret != KNOT_EOK) {
-		return ret;
-	}
-
-	return KNOT_EOK;
+	return zone_flush_journal(conf, zone, true);
 }