Mirrors the same gemini-flagged inconsistency on the walker side as
the Phase 6 PR fixed on the replay side: streaming uses "RPC_ERROR"
for transport-class failures, so the walker should too. Also folds
the nil-response case under RPC_ERROR rather than a new label —
operationally it's the same class of failure (server returned no
usable response), no point splitting the metric for it.
Chris Lu
ed5fa1926d