Standardize all error messages

[georgevanvliet]

Suggested format:

{
  "error": "some_immutable_error_enum_string_so_we_can_differentiate",
  "message": "This could contain some detail, and can be different for each invocation",
}

No need to add the http status code, since that's already in the response.

If you want to keep the possibility open for multiple errors, if applicable, you can decide to always return an array of errors such as:

[
  {
    "error": "some_error",
    "message": "some message",
  },
    "error": "some_other_error",
    "message": "some other message",
  },
]

But if you do, please always return errors an array for all calls, even if there is only one error.