Fixed #14223 -- Extended unification of exception raised in presence of integrity constraint violations.

The unification had been introduced in r12352 and native backend exceptions still
slipped through in cases that end in connection.commit() call. Thanks Alex,
Jacob and Carl for reviewing.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14320 bcc190cf-cafb-0310-a4f2-bffc1f526a37