agent: cache notifications work after error if the underlying RPC returns index=1

Fixes #6521

Ensure that initial failures to fetch an agent cache entry using the
notify API where the underlying RPC returns a synthetic index of 1
correctly recovers when those RPCs resume working.

The bug in the Cache.notifyBlockingQuery used to incorrectly "fix" the
index for the next query from 0 to 1 for all queries, when it should
have not done so for queries that errored.

Also fixed some things that made debugging difficult:

- config entry read/list endpoints send back QueryMeta headers
- xds event loops don't swallow the cache notification errors