Fix localStorage issues for IE

Summary: IE would break when DOMStorage was off due to behavior of window.localStorage

Test Plan: Open up IE, turn off DOMStorage in advanced settings, make sure console throws no errors and the warning to enable DOM Storage shows up.

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6863