Fixed #2377 -- Imported ImproperlyConfigured exception into django.auth.models.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3376 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/auth/models.py

 from django.core import validators
+from django.core.exceptions import ImproperlyConfigured
 from django.db import backend, connection, models
 from django.contrib.contenttypes.models import ContentType
 from django.utils.translation import gettext_lazy as _