Add a utility function that wraps `fetch` to throw more consistent and
useful exceptions if the request fails, making downstream handling
easier.

- Error messages thrown by failed `fetch` requests are unhelpful for
  downstream consumers. There is no specific Error subclass and the
  error messages vary depending on cause and browser.

- Parsing the JSON response from a request with `Response.json()` can
  fail if the request has a success status and the error messages also
  vary by browser.

The `fetchJSON` utility throws a `FetchError` error if either of the
above failures occurs or if the response code indicates a failure.