Any message that is posted because of a system event - whichever you can think of. Currently only a failed asynchronous upload is notified as far as I know.
"Leave" should be translated as a verb, as in "To exit", "To go away"?
Verb ar not verb isn't an issue for me. But the verb "leave" could mean exit, e.g. exit a program or add, e.g. if somebody calls another person by phone and this other person is not persent, the caller can leave a message to the other person to inform him about something. What sense is meant here?