requests failed "above" EDS level should be reflected in load reporting

There are at least a couple of cases where Envoy will fail requests "above" the EDS layer:

- When the EDS resource does not exist.
- When the cluster fails requests due to [maintenance mode](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/router_filter.html?highlight=maintenance%20mode#runtime).

In these cases, the failed requests don't show up in LRS load reporting at all. I think this should be changed to report these as failed requests in an [UpstreamLocalityStats](https://github.com/envoyproxy/envoy/blob/7aec38f986aa6b74ec04865ebc2264d599f1ba94/api/envoy/config/endpoint/v3/load_report.proto#L26) message that does not have the locality field populated. This representation seems compatible with how we report stats for non-EDS clusters (e.g., LOGICAL_DNS clusters).

CC @htuch