Fixed #16935 - misleading message if AttributeError escapes during SimpleTemplateResponse.render

Thanks to isagalaev for the report.

As discussed on django-devs, this reverts some of the changes in [16568]
i.e.  the addition of `SimpleTemplateResponse.__getattr__`, because this
makes it much harder to debug the common case of an AttributeError somewhere
during the rendering of a SimpleTemplateResponse.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16917 bcc190cf-cafb-0310-a4f2-bffc1f526a37