Fix crash when localStorage is disabled

Summary:
This still seems to be breaking people, and I want to set it back to the old
behaviour for the time being

I'm not sure why this issue report is still seeing this in non-private mode,
but we might as well make KA usable for them:
https://code.google.com/p/khanacademy/issues/detail?id=29475

Test Plan:
substate and load addition-1 in safari private mode
make sure you can answer a problem correctly

Auditors: alpert