Provide a more informative alternative to 404 on invalid shortened Phurl URL

Summary: When accessing an invalid URL on the short Phurl domain, users should see informative message

Test Plan: Open URL in the previously configured Phurl short domain such as https://www.zz.us and see dialog with message. Open https://www.zz.us/u/123 for a valid U123 Phurl and access destination URL.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14450