Currently, such a convoi will wait infinitively, getting the game to a standstill.
My suggestion is the reverse: When all destinations in a schedule are overcrowded, then the loading limit is ignored and the convoi leaves immediately.
This will probably have the effect that the loading limit was intended to prevent. Namely, unnecessary empty journeys that can cause high costs.
I tend to prefer a message that cannot be switched off and that points this out.