fix(ras-acc): update limited subscription cart error and modal layout

[chickenn00dle]

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Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1207878733150171/f

This PR updates the limited subscription checkout error message and adds a back button to the modal when an error is present:

How to test the changes in this Pull Request:

1. Set up a limited subscription product
2. Add a checkout button for the limited subscription to any page or post
3. As a reader that does not have a subscription for this product, purchase the limited subscription. Confirm you can checkout with no issues and there is no additional Go back button present in the checkout modal.
4. As the same reader, attempt to purchase the limited subscription once more. Confirm an error appears in the modal and a Go back button is present as pictured above
5. Confirm the Go back button closes the modal
6. Repeat step 4 and this time confirm the X also closes the modal

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?

[laurelfulford]

Work as described, the 'Go Back' button is a nice touch!

Absolute non-blocking question: should these strings be included with the others that can be filtered?

[chickenn00dle]

Absolute non-blocking question: should these strings be included with the others that can be filtered?

That's a good question. I don't think we are filtering any other RAS errors so I didn't include this one. If we'd like to though, I think we can tackle that in another PR where maybe we make all reader facing RAS errors filterable?