Don't fatal on image macro list if a file has been deleted

Summary:
We added the ability to delete files a while ago, but this interface isn't happy about it.

I still render the macro so you can see/delete it, e.g.

Test Plan: Viewed a deleted macro page, got a page instead of an error. Also verified that the actual remarkup part doesn't have issues.

Reviewers: btrahan, vrana, jungejason

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2328