dont show unauthorized messages for deleted files

fixes CNVS-11962

students who look at files that have
been deleted by the owner shouldn't see an
unauthorized message, it makes them uncomfortable

instead this provides them with a "this file
has been deleted" message.

refs CNVS-15815

TEST PLAN:
-login as a teacher
-add a files to the courses files area
-create a page, publish it, and link to the file
-delete the file
-login as a student to the same course
-go to the created page
-click the link
-you should see a 404 page that says "This
  file has been deleted"

Change-Id: I702c20a27c6ef2425d53298bdb4acd9e4a53435e
Reviewed-on: https://gerrit.instructure.com/41883
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>