Let PhabricatorPlatformSite catch unconfigured requests

Summary: Fix T8717. If the install didn't configure base-uri, assume they want Phabricator; We'll later show the setup warning about it.

Test Plan: Set base-uri to something else, see short error. Delete it, see Phabricator.

Reviewers: laomoi, #blessed_reviewers, epriestley

Reviewed By: laomoi, #blessed_reviewers, epriestley

Subscribers: laomoi, epriestley, Korvin

Maniphest Tasks: T8717

Differential Revision: https://secure.phabricator.com/D13482