fix file preview for hidden and nonexistent files

test plan:
 1. enable "better file browsing"
 2. go to the files page
 3. click the cloud icon next to a file, select "Restricted Access",
    and then "Only available to students with link".
 4. as a teacher, attempt to preview the file. you should see the
    file, not a not-found error.
 5. attempt to preview a nonexistent file by going to the URL
    /courses/X/files/Y/file_preview
    (where X is a *valid* course ID and Y is an *invalid* file id).
    you should see an error message without Canvas chrome
    (no header or left-side menu)

fixes CNVS-16394

Change-Id: I9c632d664e9fd8758dc63b21e6751a914f4a4ceb
Reviewed-on: https://gerrit.instructure.com/43175
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>