Lunch Prodigy minor fixes

We made some minor fixes to some annoyances when using Lunch Prodigy.

• When the order has already been taken and the user tries to delete an order item, the user is now informed that he cannot delete anything from an already taken order.
• When the user tries to delete a non-empty order, an error is displayed that all orders must first be deleted.
• When the user tries to add something to the order, and the order has already been printed, he will be warned that he should inform the person who has printed the order to make sure he gets what he wants.