404 if the Repo can't find a document

With the introduction of a Repo for fetching documents, the Repo would throw an error if params[:section_id] didn't match a placeholder file instead of allowing the controller to respond with a 404. This commit checks for the existence of a file first, which if false allows the controller to respond with a 404.
This would match the rest of the test language better if it were something like `Then I should get a page not found response`