Update README

[aparticka]

Fixed description of error handling

[gbirke]

Please merge this. I was very puzzled by this until I had a look at the code.

[ecstasy2]

This one should be merged I think.
Just spent 30min thinking my code was in fault while actually the api has changed.

[jimf]

+1, was about to submit the same fix. Though 1 minor nit: the error case also sets payload to the error result.