[feedback] add more warnings to log_listener to help debug flake

the most recent flake instance revealed that LogMany() was never called
with no messages, but Done() was indeed called when logs_ is still
empty. These new warnings will help us figured out if Done() was called
before any calls to LogMany() or if it might be some race between
LogMany() and Done().

FLK-179 #comment

TESTED=`fx run-test feedback_agent_tests -t log_listener_unittest -- --gtest_filter=CollectSystemLogTest.Fail_LoggerNeverCallsLogManyBeforeDone`

Change-Id: I1954d9e55247cc21b6e07a713ba700cd877f021c