MADS: Add a sanity check for color 255

This is used in scene 752
MADS: Add a sanity check to scene 413
MADS: Document all game scenes, and allow entering unimplemented scenes

This way, it's possible to enter all scenes for debugging purposes.
Also, this avoids the scene reordering during development. Scenes 708
and 806 have been removed, as they don't exist