Handle entry/exit doors on invalid surfaces

[vrad-exe]

There is one edge case that can result in doors being moved to invalid surfaces within the editor: if a style has floor/ceiling corridors for singleplayer but not co-op, the singleplayer door is moved onto the floor/ceiling, then the mode is switched to co-op, you now have a co-op door on the floor/ceiling despite not having any valid corridors for that surface.

• Compiling will produce an error display with the message No corridors available for the <x> group! This should be changed to a more user-friendly message which uses nicer names for the groups (e.g. "singleplayer entrance door on a wall" instead of "Horizontal_Sp_Entry") and extra text explaining how it got into that state and how to fix it. (Could also enable the chamber display with the position of the door highlighted.)
• Moving the door back onto a wall can result in it being sideways, since DesiredFacing only gets added when there are valid floor/ceiling corridors. It should be present always, not really sure why it was made conditional to begin with.

nln2fORAdO.mp4

[vrad-exe]

It took me until now to realize this can also just happen when switching styles, if the new one doesn't have any corridors for that surface. The error message should probably say that it can happen if you recently switched styles or between singleplayer/coop, e.g:

No corridors found for the <groupname> group. If you recently switched styles or switched between singleplayer and co-op, you may need to move one of the entry/exit doors to a different surface.