Unity In-App-Purchase Best practice

Hi, I am adding In-app-purchase into my project by using Unity Built-in purchasing API. Following the tutorial, in function ```ProcessPurchase()```, I have to return ```PurchaseProcessingResult.Complete``` after successfully give user an item. What if there an exception occurs before I can give user an item, and that causes ```ProcessPurchase()``` do not return anything. How could I restore that purchasing when launch game again? Thank you,