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Purchase Bug

There's a bug in the merchant system that allows players to buy items from the shop even when they don't have enough gold.

Assignment

Complete the purchase_item function.

If the character doesn't have enough gold, raise an Exception with the text not enough gold.
Otherwise, return the amount of remaining money the customer has after completing the purchase.

Raise the exception but don't worry about handling it; that's up to the caller (i.e., the tests).