no issue
- upgrade `ember-ajax` to 3.0.0
- `ember-ajax` [now passes the payload through directly](https://github.com/ember-cli/ember-ajax/releases/tag/v3.0.0) rather than trying to normalize it so all our error handling needed to be updated
closes https://github.com/TryGhost/Ghost/issues/8203
- rename `version-mismatch-test.js` to the more generic `error-handling-test.js`
- add tests to simulate receiving HTML errors (eg, the type of error received from CloudFlare for a gateway timeout)