Add error handling for 406 error

```
Jan 31 21:27:22.990 DEBUG rover: error=RoverError { error: Failed to validate schema

Caused by:
    encountered a GraphQL error, registry responded with: 406: Not Acceptable, metadata: Metadata { suggestion: None, code: None } }
error: Failed to validate schema

```

Note: I ran into this trying to run checks with an invalid api key

