Fix network FSM errors when a client closes the game during the loading screen or rejoin synchronization stage, fixes #4594, refs 3199.

Fix comments claiming to ensure reliability when in reality there is only a small likelihood of the message being received.

Broken by rP14732 / refs #2420 which was a preparation for rP14772 / refs #2373, which hence didn't actually work and was circumvented by rP15106 without clarification.
Might have caused #3643.
Differential Revision: https://code.wildfiregames.com/D1557
Reviewed By: Imarok