Fixed #8136: Added a signal emission when an error is raised handling an...

Fixed #8136: Added a signal emission when an error is raised handling an error. This was required for the test client to handle missing 404.html templates and errors in the 404.html template. Thanks to danfairs for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8464 bcc190cf-cafb-0310-a4f2-bffc1f526a37