Fix a fatal during breadcrumb construction when viewing a dashboard you don't have permission to view

Summary: Ref PHI662. Viewing a dashboard you don't have permission to view (in the Dashboard application) currently fatals while building crumbs, since we fail to build the ... > Dashboard 123 > ... crumb.

Test Plan:

• Viewed a dashboard I didn't have permission to view in the Dashboards application.
• Before patch, fatal when calling getID() on a non-object.
• After patch, sensible policy error page.
• Viewed a dashboard I can view, saw sensible crumbs.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19449