All HTTPRoutes marked as invalid when config apply fails

[sjberman]

As described in #3856 and in a comment, when a config apply fails for whatever reason, it seems that all HTTPRoutes are marked as invalid, which can cause disruptions, especially if agent successfully rolls back the configuration to a working version. For example, external-dns will notice that HTTPRoutes are marked as invalid, and remove DNS entries.

While our control plane needs to continue trying to apply the latest config, we should investigate how to improve this experience and not mark all Routes as invalid (or more specifically Accepted: false) when a config apply fails.

Dev notes

• If the Route is syntactically valid but the config apply fails for some reason, we should have Accepted: true on the Route but have Gateway not programmed or similar