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.

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 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