Fixed #10841 -- Switched response served when DEBUG=True and request.is_ajax()...

Fixed #10841 -- Switched response served when DEBUG=True and request.is_ajax() returns True (indicating request has been generated by a JS library) to a plain text version for easier debugging.

Contents of this response are similar to its HTML counterpart modulo frame variables values in the Python traceback section.

Thanks to Riz for the report, to SmileyChris for the patch and to Julien for reviewing.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16921 bcc190cf-cafb-0310-a4f2-bffc1f526a37