Add error handling to checkout process

[seer-by-sentry]

Fixes REACT-5FV. The issue was that: Backend process_order raises unhandled Exception for insufficient inventory, causing Flask to return a 500 Internal Server Error.

• Added a try-except block to the checkout route to catch any exceptions that occur during the process_order function.
• If an exception occurs, return a 400 error with a JSON response containing the error message.

This fix was generated by Seer in Sentry, triggered by [email protected]. 👁️ Run ID: 318968

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