Better error message when deserializing bad root.json

We're trying to configure `stack` to use a private Hackage instance, and we kept seeing the message:
```
Could not deserialize /root.json: Schema error: Expected field "signed"
```
Adding `trace` statements and building `stack` manually, we were able to find out that it was attempting to parse
```json
{
"errors": [
{
"status": 401,
"message": "Authentication is required"
}
]
}
```
It would be nice if the message could show, in some way, the JSON file that failed to be parsed