Improve "Host" header check

Summary: See IRC. This check is somewhat misleading right now because it could arise from a mangled/broken Host header rather than a bad phabricator.base-uri configuration.

Test Plan: Faked this to trip, read all the text.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11894