return 500's for invalid HTTPRoute backend refs #162
Description
There is a set of rules around how to handle 1+ invalid backend refs on an HTTPRoute that generally involve returning 500 responses for any invalid backends. This needs to be implemented properly.
https://gateway-api.sigs.k8s.io/references/spec/#gateway.networking.k8s.io/v1beta1.HTTPRouteRule (see backendRefs).
Needed for core conformance.