R1:2b5bb642c075

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…