With this function you can make your game go to the previous one
as listed in the resource tree at the time the game was compiled.
if this room does not exist, and error will be thrown and the game
will be forced to close. Note that the room will not change until
the end of the event where the function was called, so any code
after this has been called will still run.
if room_exists(room_previous(room)) room_goto_previous();
The above code will check to see if there is another room before the current one and if so it will go to that room.