From the examples it is expected that obj['error'] is always set #25

Hi, thanks for this great library. I just wanted to report a behavior that, in my opinion, is not very obvious: in the examples it is shown that to check for errors it's enough to check whether obj['error'] is True. But when the request is successful the key 'error' is not set at all which might lead to a KeyError.

Is there a reason why obj['error'] is not set to False if the request is successful instead of not being set at all? I would expect this behavior for error_code not for just error.

Thank you.