Exception caused by RTDS misconfiguration not caught by validation server

We saw a case in which a misconfiguration at the RTDS layer was not caught by the validation server. In staging (running the server in its normal mode) Envoy crashed with the following exception as expected:

`EnvoyException in c-ares callback: envoy.config.core.v3.ApiConfigSource must have a statically defined non-EDS cluster: 'someclustername' does not exist, was added via api, or is an EDS cluster`

Ideally this sort of error could be caught in development via running Envoy in validation mode. I've not looked too much into the validation server yet but the exception is thrown after `starting main dispatch loop`, which I noticed is not part of the validation process today based on the logs I'm seeing.