Limit damage caused by bad panels on dashboards you can manage
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.
Reviewed By: chad
Maniphest Tasks: T12203
Differential Revision: https://secure.phabricator.com/D17311