I quite like having customised messages returned for debugging purposes (eg: 'Duplicate domain name exists' instead of 'Unprocessable entity'). It'd be even better to have a formalised error type returned from the API, although this isn't really within the scope of these changes... eg:

```json
{
    "error": {
        "status": 422,
        "message": "Duplicate domain name exists"
    }
}
```