Standardise errors

[hiett]

Cases to test:

• Missing bearer header, eventually hits a match for :malformed_data, so :not_found is never actually triggered (can remove if so) It was unused, but is now being used for actual 404 errors such as below
• Handle additional 404s as JSON errors too, as could be due to future Upstash features using a new endpoint (such as when multi-exec was added)
• Not including Content-Type json will cause a 500 to be thrown without an error message at all SRH code isn't even hit, this is happening by a missing Plug Parser