Fixed #15158 -- Fixed error introduced in r15252 in rendering of the template post morten section of the 500 error debug page with loaders other than the file system or application dir loaders. Refs #15122. Thanks gsf for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15360 bcc190cf-cafb-0310-a4f2-bffc1f526a37