Temporary fix for crashing on exit.

Okay, speculation as to why it crashes. its referencing a material that
dont exist when u press exit BECAUSE you delete the marble before u
cleanup the server.  So...since this only happens on leave, we can
PROBABLY ignore for AT LEAST single player.  dunno about MP.  if we can
ignore it for both, then just take out warning and keep continue, and
don't worry about it.