Add support for defining response errors in specs

[matomesc]

For example consider the facebook api errors here.

We can easily map non-200 responses to create errors from the response body.

@ttezel should the callbacks be invoked with (err, res, body) where err would be an error with error message and body is the parsed response body, or attach the response body on the err object (err.body) and callback with (err, res)?