Mission 3 : adding initial position and error messages

[fbouchez]

Adding init.sh to mission #3 so we start from the cellar (end position of mission #2).
Otherwise, after a restart we are already in World/ so the first goal is a bit strange.
Also added a message stating the current position when starting/resetting the level.

Now accepting also cd ~ (before, was only cd) for the first goal, to go back to the starting point.

Added error messages to give more information after seeing some students struggle with this mission :

• stating the command is too complicated if using e.g. cd ../../
• explaining the previous to last command must go back to starting point
• error message if current position is not in the throne room.