Fixes for xlsx Content-Type

1. django-tables2-reports throws 500 Sever Error when report format is not recognized. 404 is more appropriate in this case.
2. django-tables2-reports sets Content-Type to application/vnd.ms-excel for xlsx files which causes warnings in Firefox. application/vnd.openxmlformats-officedocument.spreadsheetml.sheet is the correct Content-Type for xlsx