[feedback] add warnings to log_listener to help debug flake

given that we don't see the warnings regarding the timeout in the flaky runs,
these new warnings would help us figuring out why the logs are empty: is LogMany()
called with no messages? is Done() called whith logs_ empty?

FLK-179 #comment

TESTED=`fx run-test feedback_agent_tests -t log_listener_unittest`

Change-Id: I11ffb1d9efad8c12a136140405e3b5aca51c9c54