Currently if we query the Prometheus HTTP API with an invalid query, or there are other errors, the run_command or query_range methods just return nil. E.g. for the response

{"status": "error", "error": "1:6: parse error: unexpected identifier \"a\""}

This PR ensures we check the status field in the response body, which should be success on successful responses, and raises RequestError otherwise. Based on https://prometheus.io/docs/prometheus/latest/querying/api/.

I think some way of distinguishing good requests/responses from bad is nice, and given that there's already an exception here, perhaps we can reuse it?

@yaacov