Partial fix for owners bug where a package's repository doesn't exist.

Summary: Avoid requesting a non-existent repository...

Test Plan: Delete a repo that has an associated owners package. Then verify that the owners list page no longer throws an exception.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1372

Differential Revision: https://secure.phabricator.com/D2776