Unhandled sessionStorage QUOTA_EXCEEDED_ERR JavaScript errors

Steps to reproduce:

1. Use Safari in private browser mode or configure your browser to disallow local storage (see screenshot).
2. Install GreenHopper and go to the TaskBoard.
3. Move a card to a new state.

Result: A JS error (QUOTA_EXCEEDED_ERR, line 21 in persistence.js) is thrown after the ajax request completes, which leaves the page in a bad state – e.g. dialog doesn't close. (See screenshot attached.)

Expected result: QUOTA_EXCEEDED_ERR errors should be handled with a try/catch around localStorage/sessionStorage setItem() calls. (example)

Workaround: Use Safari in non-private mode (i.e. normal mode) or re-enable local storage.