handle file not found case when previewing a file

fixes CNVS-16417

When previewing files, if you click on a preview file link that is try
to preview a file that has been deleted or doesn't exist, intead of
doing nothing and showing the first preview in the series of previews,
it will now show a "File not found" messgage that explains why

Test Case

Given you have new files enabled
And you've uploaded a few files
And you've click on a file to show it's preview
When you copy the url for that file preview
Then you delete the file you saved the url
And you try to go to that url after the file has been deleted
Then you should see a preview page that says "File not found"

Change-Id: I8e0bb43cc12287dc032b8e0cf62dc7b3a4775431
Reviewed-on: https://gerrit.instructure.com/45736
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Sterling Cobb <sterling@instructure.com>