Suppress stats streaming error log messages when task finishes.
Streaming errors are expected when a task finishes and they aren't
actionable to users.

Also, note that the task runner Stats hook retries collecting stats
after a delay. If the connection terminates prematurely, it will be
retried, and closing the stats stream is not very disruptive.

Ideally, executor terminates cleanly when task exits, but that's a more
substantial change that may require changing the executor/drivers interface.

Fixes #10814