add IUserFriendlyError adapter for violation of check constraints
This way we get back the same error messages we get from the python
check.
Related to #5154406
Add some entities
-----------------
As manager, you can access to entity creation forms by using the `site management`_ page.
.. _`site management`: ../manage
As regular user, the index page or the action box may propose some links to create entities according to the context.