Fix Pholio feed fatal

Summary: need to filter images that we can't find mocks for. Fixes T3645. Note I have some other errors in my feed which are really tricky to debug and might be garbage data; I want to see what happens in prod post this push.

Test Plan: set a mock visibility to no one and feed worked

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3645

Differential Revision: https://secure.phabricator.com/D6631