[dbapi] move ProgrammingError into cubicweb module
A new ``repoapi`` will be introduced as a replacement for the dbapi. It will
need ProgrammingError too.
Related to #2503918
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.