React to Aphlict disconnects in the UI

Summary: Ref T5365. Surface disconnects in the UI.

Test Plan:

• Connected, then killed the server.
• Saw disconnected event and appropriate update in the UI.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5365

Differential Revision: https://secure.phabricator.com/D9706