DLTP-250 BUG - 500 error on accessing a restricted download

Download controller did not have default layout set and So on error
application did not know how to handle then throwing exception as
"There was no default layout for DownloadsController...."
This change fix that and throws the appropiate error code
Merge pull request #64 from ndlib/DLTP-250

DLTP-250 BUG - 500 error on accessing a restricted download