Fixed #12001 -- Modified the example of form validation to use the form's...

Fixed #12001 -- Modified the example of form validation to use the form's error class, rather than assuming ErrorList. Thanks to Suor for the report, and timo for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13221 bcc190cf-cafb-0310-a4f2-bffc1f526a37