fix(modal-checkout): move checkout_error trigger

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The checkout_error may not always run because the script might end up running after the DOMContentLoaded event dispatch. This PR tweaks the approach so the trigger runs in the modal script, which runs inside domReady() and is ensured to always run.

How to test the changes in this Pull Request:

1. While on the epic/ras-acc branch, use the checkout button to purchase a subscription you already own (make sure the subscription product is configured to restrict such purchase)
2. Click the checkout button a few times to confirm it sometimes hangs in the loading state and sometimes goes through with the correct error message (try also throttling the internet connection in a few attempts)
3. Checkout this branch, click the checkout button several times and confirm the error message renders consistently

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[chickenn00dle]

Great catch!