Phurl URL object should show a banner if the URL is invalid

Summary: Ref T8989, Phurl URL should always show an info banner if the URL isn't valid

Test Plan: Phurl objects with URL "google.com" should show an error banner, but objects with URL "http://google.com" should not show banner.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T8989

Differential Revision: https://secure.phabricator.com/D14386