Modify API for SimpleHttpClient.get_json and update usages.

Previously, this would only return the HTTP body as JSON, and discard other
response information (e.g. the HTTP response code). This has now been changed
to throw a CodeMessageException on a non-2xx response, with the response code
and body, which can then be parsed as JSON.

Affected modules include:
 - Registration/Login (when using an email for IS auth)