Add checks and explicit errors when cart buttons are missing data

[tomalec]

Changes proposed in this Pull Request:

Add checks and explicit errors when cart buttons are missing data to help debugging issues with other extensions manipulating WC Core add & remove from cart buttons or events.

This would help identifying issues like the one reported at https://wordpress.org/support/topic/js-error-on-add-to-cart-when-logged-out/

Checks:

• [x] Does your code follow the WordPress coding standards?
• [ ] Have you written new tests for your changes, as applicable?
• [x] Have you successfully run tests with your changes locally?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Screenshots:

Detailed test instructions:

1. Build the branch
2. open shop page
3. Run synthetic/malformed cart events

   jQuery( document.body ).trigger( 'added_to_cart' );
   jQuery( document.body ).trigger( 'removed_from_cart' );

4. Check for errors in the console
5. Add an item to the cart using UI.
6. Remove all product ids document.querySelectorAll('[data-product_id]').forEach((e)=>e.removeAttribute('data-product_id')). 8. Remove item from mini cart using UI
7. Check for errors in the console
8. Smoke test not tempered events and buttons for adding to & removing from cart, for regression

Additional details:

Changelog entry

Tweak - Add checks and explicit errors when cart buttons are missing data.

[tomalec]

Thanks @eason9487 for the detailed review :)