Error handling support is not useful

[kmicklas]

apiRequestXhr and performXhrRequests use Either Text to report an error condition. However, the only possible error string ever generated is Response could not be decoded for request: plus the JSON encoding of the request.

Even if we aren't really meant to use HTTP errors for application errors, it is still useful to be able to distinguish different types of non-application errors, e.g. internal server errors, timeouts, authentication issues, etc.

As it stands, the error reporting is no more useful than Maybe. For maximum flexibility I think it would help if the actual XHR response was returned on a non-successful status or JSON decoding failure.