Tweaked a few logging levels to not spam error level logs. Also make it easy to debug issues in case relay retry fails.
[1] Put non-fatal exceptions that should be logged at warning in their explicit sets. Also always use the root cause to determine if an exception is non-fatal, because sometimes the actual causes are wrapped inside other exceptions.
[2] Record the cause of a relay failure, and record if a relay retry is successful. This way we can look at the log and figure out if a relay is eventually successful.
[3] Add a log when the frontend connection from the client is terminated.
[4] Alway close the relay channel when a relay has failed, which, depend on if the channel is frontend or backend, will reconnect and trigger a retry.
[5] Lastly changed failure test to use assertThrows instead of fail.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208649916
0e64015cdf