* Changed invalid fixture from trying to insert a NULL value in a
  null=False CharField to try with a DateTimeField instead because our
  Oracle backend allows NULLs for CharFields than can be blank and that
  insert wouldn't generate an error.

* cx_Oracle raises a
  `DatabaseError: Could not load fixtures.Article(pk=1): ORA-01407: cannot update ("USER_DEFAULT"."FIXTURES_ARTICLE"."PUB_DATE") to NULL'`
  instead of an IntegrityError like the rest of the DB-API drivers we
  support, changed the error message we test for to start simply with
  'Error' to catch both messages.

  We might want to consider converting DatabaseError to IntegrityError
  in the backend when the error code is ORA-1407.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17002 bcc190cf-cafb-0310-a4f2-bffc1f526a37