logs: use %o to format error

otherwise, they are printed as multi-line and this messes up tail+date formatting

src/directoryserver.js

@@ -309,7 +309,7 @@ async function start() {
     });
 
     gServer.on('error', function (error) {
-        debug('server startup error ', error);
+        debug('server startup error: %o', error);
     });
 
     gServer.bind('ou=system,dc=cloudron', async function(req, res, next) {