Handle Paddle billing customer already exists error

[nduplessis]

Pull Request

Summary: When attempting to add a customer to Paddle Billing that already exists, an unhandled error occurs

Description: https://developer.paddle.com/errors/customers/customer_already_exists

Testing:

Screenshots (if applicable):

Checklist:

• [ ] Code follows the project's coding standards
• [ ] Tests have been added or updated to cover the changes
• [ ] Documentation has been updated (if applicable)
• [ ] All existing tests pass
• [ ] Conforms to the contributing guidelines

Additional Notes: