VCell's behavior when closing a window while disconnected is undesirable

[CodeByDrescher]

If the client is disconnected (and does not know it's disconnected) when the user tried to close a window with changes, two bad things can occur:

1. the user selects "no" (i don't want to save my changes), and VCell locks up for ~7-30 seconds; no user input registers.
2. the user selects "yes" (I would like to save), and VCell futilely attempts to save the model to a server it hasn't realized it's disconnected from.

This may be related to VCell freezing up overnight, as the logical pathways may be similar.