Almost no one is aware that rejoining multiplayergames is not supported.
In rP19491 an explanation was added in case someone tried to rejoin a multiplayer game with AIs.
This patch should do the same for the gamesetup and prevent players from wasting their time or only play if they know that their connection will be stable.
To not be pesky if the player doesn't care about the warning, should it a most once per gamesetup.
To keep the player reminded of this fact, display it at least once per MP gamesetup when trying to ready up with AIs being assigned or launching such a game.