limit_validation_errors fails on checkout with cart and coupon handler

Since coupon form is available in cart thanks to views handler (https://www.drupal.org/node/1684892), if you display cart view with coupon handler in checkout page (thanks to commerce_cart_form_checkout_pane module), coupon submission will fail.

#limit_validation_errors only take care of coupon_code value, but in this case, coupon_code is in a tree 'cart_contents_form', 'cart_contents_form_view', 'coupon_cart_form'. So in case commerce_cart_form_checkout_pane is enabled, we should take care of this tree.

How to reproduce

1. Install commerce_cart_form_checkout_pane module
2. Add cart pane in checkout/*/checkout
3. Add coupon form view handler in cart view
4. Submit a valid coupon in coupon form in checkout page.
5. Coupon always fail