Improve error messaging for empty Conpherence threads

Summary: Fixes T7275. This makes the error stuff a little more consistent with other modern UIs.

Test Plan: {F307286}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7275

Differential Revision: https://secure.phabricator.com/D11778