knotd: print error when unable to change owner of logging file

relates #728

src/knot/common/log.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
@@ -382,6 +382,7 @@ int log_update_privileges(int uid, int gid)
 
 	for (int i = 0; i < s_log->file_count; ++i) {
 		if (fchown(fileno(s_log->file[i]), uid, gid) < 0) {
+			fprintf(stderr, "Failed to change owner of logging file\n");
 			return knot_map_errno();
 		}
 	}