Show exception title as page title for AphrontUsageException

Summary: Fixes T7273. This shows a better title (like "No Such Instance") instead of a generic one ("Unhandled Exception") when the user hits an AphrontUsageException.

Test Plan: Visited a nonexistent instance, got a nice title.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7273

Differential Revision: https://secure.phabricator.com/D11771