Fixed #12702 -- Introduced a common implementation of DatabaseError and IntegrityError, so that database backends can (re)raise common error classes. Thanks for Waldemar Kornewald for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12352 bcc190cf-cafb-0310-a4f2-bffc1f526a37