Made dictsort and dictsort reversed template filters fail silently

when passed list of things that aren't dictionaries.

Thanks Harris Lapiroff for the report and Daniel Barreto for the patch.

Fixes #15652.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17374 bcc190cf-cafb-0310-a4f2-bffc1f526a37