xds: suppress the duplicate errors at the client side (#9185)

Fixes #8994

This change suppresses duplicate errors on the xds client side by
skipping calling watchers for the duplicates. This also prevents
generating new child names for the same broken EDS resource (EDS
resource has no localities) that is being re-send by the control plane.

RELEASE NOTES:
- xds: handle receipt of duplicated errors at the client side by
suppressing them in the same manner the duplicate updates are suppressed