fix: Handle payment gateway refund errors in admin

[sentry]

This PR adds a try-except block around the refund_order call in the Django admin refund view (ecommerce/admin.py).

Previously, if the underlying payment gateway (e.g., CyberSource) returned an unexpected error response format (such as a 401 Unauthorized error which lacks a top-level 'reason' key), it would lead to a KeyError and an unhandled 500 error in the admin interface.

This change catches any exceptions raised by the payment gateway during a refund attempt and displays a user-friendly error message to the admin, preventing a server error and providing better feedback. It also logs the full exception for debugging purposes.

This change complements a fix in mitol-django-payment-gateway that makes the perform_refund method more robust against varied error response structures from CyberSource.

Fixes MITXONLINE-6WC

[github-actions]

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