Update CheckoutScreen.tsx to handle out of stock errors

[seer-by-sentry]

Fixes MOBILE-REACT-NATIVE-1. The issue was that: Backend inventory validation failed, causing a 500 error during checkout.

• Display a specific error message when items are out of stock during checkout.
• Parse the error response to identify out-of-stock items and display them in the error message.

This fix was generated by Seer in Sentry, triggered by simon.zhong@sentry.io. 👁️ Run ID: 604330

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