See #1532 for more context.

- Errored JSON requests will get back the error in JSON instead of using
  the status text. This seems better to me because it seems more correct
  to utilize the response body over hijacking the status text. The
  caller is expecting JSON anyway. Worst of all I never actually set the
  status text like I thought I did so it wasn't working to begin with.
- Allow the update error to propagate for JSON update requests. It was
  caught to show the error inline instead of an error page when using
  the update page but for JSON requests it meant there was no error and
  no error code so it looked like it succeeded.
- Make errors for failed requests to GitHub less incomprehensible.
  Previously they would just be the code which is no context at all.