Don't fatal when viewing a moved document if the target does not exist or isn't visible

Summary: Fixes T5156. If a document has been moved but the new one does not exist or can't be seen by the viewer, render a generic message.

Test Plan: Viewed moved-plus-visible and moved-plus-nonvisible documents.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5156

Differential Revision: https://secure.phabricator.com/D9254