utils: should output the err.stack

```
12 Jun 16:01:30 - 744ms TypeError: Cannot call method 'destroy' of null
12 Jun 16:01:30 - 773ms connecting 107.170.192.119:8388
12 Jun 16:01:31 - 32ms connecting 107.170.192.119:8388
12 Jun 16:01:32 - 451ms connecting 107.170.192.119:8388
12 Jun 16:01:32 - 481ms connecting 107.170.192.119:8388
12 Jun 16:01:33 - 549ms TypeError: Cannot call method 'destroy' of null
```

It's hard to trace the real error, then we just output the error.stack if exists, then we/users could collect more information, and post the postmortem here.