pubsub: streaming connection logs too aggressively

[pongad]

First reported here.

The connection logs a warning even though the subscriber is shutting down. When shutting down, this error is normal. The subscriber waits to process all messages, but by the time this happens, another message might have already arrived. Trying to ack this message could error: it races with us trying to shutdown streaming channel.

This is sub-optimal but "OK". The message will be redelivered to another machine. The fix for this is to not log such errors when the the subscriber is shutting down.