Description of the Change

Re-raised in favor of PR #131 (as it was merged to the enhancement/82 branch). Please check more info on #131.

Closes #117

How to test the Change

1. Subscribe to the Mailchimp list.
2. Unsubscribe from the list using the link below the "Subscribe" button in the form.
3. Try to re-subscribe using the form with the same email and verify that the form displays an error message informing the user about the issue and providing a link to subscribe via the hosted form.

Changelog Entry

Fixed - Display an error message informing users that they must subscribe through the hosted form if they were previously unsubscribed.

Credits

Props @MaxwellGarceau @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.