Fixed #14972 -- Ensure that the HTML email logger always produces useful output, regardless of whether it has been given an exception or a request. Thanks to jamstooks for the report, and bpeschier for the initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15383 bcc190cf-cafb-0310-a4f2-bffc1f526a37