Pass back additional information to the client when there is an active record error

Changes this response 
```json
{
    "error": {
        "code": "unprocessable_entity",
        "message": "Validation failed: Name can't be blank, Description can't be blank"
    }
}
```
to
```json
{
    "error": {
        "code": "unprocessable_entity",
        "message": "Validation failed: Name can't be blank, Description can't be blank",
        "details": {
            "name": ["can't be blank"],
            "description": ["can't be blank"]
        }
    }
}
```
This is useful because the client may want to update specific fields with validation errors and not post a big blob of text under the form.