Fix duplicate entry catch for allowed address pairs

Fix duplicate entry catch for allowed address pairs

If None is submitted as a MAC address in an allowed_address_pair,
the port MAC will be used. So if two entries are submitted with the
same IP and one's MAC is None while the others is the port's MAC,
they will pass the API duplication check and fail to insert into the DB
due to a unique constraint violation.

This patch catches the db error and turns it into the same exception
the API uses on duplicate entries.

Closes-bug: #1373756
Change-Id: Ide995810d6fe0481d3add206bf0674cbbde7f05f