Handle unauthorized user error for creation form

[jmikola]

Added homepage link to creation form, but the form can error out if the user is unauthorized. An "Unauthorized" error will be thrown in the backend trying to submit the form. We need to handle this gracefully in Backbone.

For now, a general error appears above the form, but no fields have errors.

This might also affect the edit form (though that wouldn't appear to the user as easily).