Check view policy, not join policy, when raising an error for Phame custom domains

Summary: A user in IRC hit this; this looks like a typo.

Test Plan: Created a new blog with a public view policy, a non-public join policy, and a custom domain.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: legien, epriestley

Differential Revision: https://secure.phabricator.com/D8980