Use the default skin if a blog has a set but no-longer-available skin

Summary: I set one of my blogs to "phacility.com" based on arc patch and it now fatals since that's not a valid class anymore. :P Recover from these cases.

Test Plan: Viewed blog, no missing symbole exception.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3692