fix rpc recorder to:
 - assert that response is not only non-5XX but also contains correct response
 - alert on too many incorrect responses

It looks like the 400 responses are already treated as errors. My guess now is that the routes it was hitting ('get_server_time', 'ping_db') must not have actually been getting 404 responses?

See: https://chromium.git.corp.google.com/chromiumos/third_party/autotest/+/master/site_utils/rpc_flight_recorder.py#253

Slight mystery, holding open 1 more week.

likely fixed by other means (improved shard_client alert, others...)