Improve error handling

[forste]

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