Raise a better error in Phame when trying to set two blogs to the same domain

Summary: Currently the exception escapes to top level. Instead, intercept it and complain.

Test Plan: Tried to set two blogs to the same domain.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3701