I don't think we want this change. An error response status could still contain a JSON body. For example, here's an [error response](http://developer.github.com/v3/#client-errors) from the GitHub API.

```
HTTP/1.1 422 Unprocessable Entity
 Content-Length: 149

 {
   "message": "Validation Failed",
   "errors": [
     {
       "resource": "Issue",
       "field": "title",
       "code": "missing_field"
     }
   ]
 }
```