Fixed #15628 -- Made the admin error email more useful when there isn't a...

Fixed #15628 -- Made the admin error email more useful when there isn't a request associated with the message. Thanks to hynek for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15869 bcc190cf-cafb-0310-a4f2-bffc1f526a37