Improve error handling

hey,

thanks for you work so far. 

However, I'd appreciate some proper error handling, at best in a (de-facto) standard way, i.e. calling callbacks like

```
cb(error)
```

when an error occured,
or

```
cb(null, result)
```

when everything is alright and we just want to pass the result.

I can change that myself but would like to know your feedback first
