Fixed inconsistent error response from the API
Fixed inconsistent error response from the API

- we have two JSON error response formats one old, one new (v2)
- we couldn't use the new one everywhere before without changing the response from older versions
- that is totally irrelevant in Ghost 5.0 as there is only one API version
- therefore we can and should use the new response format everywhere
- eventually we should rename it so it doesn't have v2 in it

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost