Fixed order - product deletion conflict

Fixed by adding placeholder product, upon deletion of a real product its
place in orders is replaced by this placeholder. Please note additions
to config.php - two new constants PLACEHOLDER_CAT_ID and
PLACEHOLDER_PROD_ID. In my test database I created a placeholder product
with pr_id 0 and placeholder category with the same id. The placeholder
category is only relevant because I don't want the placeholder product
blocking delete operation on real categories.

Also improved category deletion error message.