Limit damage caused by bad panels on dashboards you can manage

Summary:
Fixes T12203. If you tried to manage a dashboard which had a panel you can't see, we'd try to render bogus actions for it and fatal.

Instead, for the moment, survive. Presumably we'll ship a real fix for this in the next release or so, and tackle T10612 / T10145, which I think are closely related.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12203

Differential Revision: https://secure.phabricator.com/D17311