Better error messages.

There is an error that comes up (seemingly) for multiple scenarios with Registration/Commerce Registration:

This product is not available for registration at this time.

It can mean a handful of things.

• User does not have permission to register.
• Registration is not enabled.
• Registration has no slots left.
• Registration open date has not been reached.
• Registration closed date has been reached.

// Check that registrations are open and the user has access.
    if (!registration_status('commerce_product', $product_id, TRUE) || !registration_register_page_access('commerce_product', $product)) {
      form_set_error('product_id', t('This product is not available for registration at this time.'));
      return FALSE;
    }

I think, for a better end user experience, this should be clarified and broken down, for example, something like:

if (!registration_status('commerce_product', $product_id, TRUE)) {
      form_set_error('product_id', t('We're sorry, but you cannot register at this time.'));
      return FALSE;
    }

if (!registration_register_page_access('commerce_product', $product)) {
      form_set_error('product_id', t('We're sorry, but you do not have permission to register for this (event?). '));
      return FALSE;
    }

Also, if we know before hand that the product fails both checks above, 'Add to Cart' form shouldn't even be shown.

It's somewhat confusing for users right now to get such a generic message, and also difficult to debug since it can mean a variety of things.