change error msg on a bad request for the Daily Form Stats report

http://manage.dimagi.com/default.asp?137662

For specifically the Daily Form Stats Report, I modify the error message that shows on a Bad Request so that it informs the user to limit their date span. 

Figured this was the best way to handle this case. Modifying the datetime module / filter to only allow a certain range of dates would take much more effort and custom additions to the filter (possibly a new filter).

Couldn't reproduce locally since my server doesn't have apache running with LimitRequestFieldSize or LimitRequestLine set. I tested locally by making GenericReportView.json_response() always return a 400