Group and app names in groups validation error.

Yields

```
HTTP/1.1 422 
Content-Type: application/json
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked

{
    "errors": [
        {
            "attribute": "apps", 
            "error": "Groups may contain apps or groups but not both! Apps: [/inky-default] Groups: [/test]"
        }
    ]
}
```

@BenWhitehead