add a file not found error message

fixes CNVS-16157

Currently if you try to go to a file link that doesn't exist, it will
just show you a page not found error. This leads people to contact
support. Now, you see an error message that tells students to contact
their instructure.

Test Case
As a teacher
Given you have exported a course, but only export a courses wiki pages into a canvas
package
And that course you exported has a link to a file in the wiki page
And you create a new blank courses
And import your export the exported course into the new blank course
Then when you navigate to the new courses wiki page
And click on a link in the wiki page
Then an unauthorized access page should appear
And that page should instruct students to contact their instructor

Change-Id: Ic926bb06c931fd61bc56bd42fe6d8b7edefeefae
Reviewed-on: https://gerrit.instructure.com/42475
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Product-Review: Sterling Cobb <sterling@instructure.com>
QA-Review: Sterling Cobb <sterling@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>