Bug 1034570 - Notify the user if initial registration fails r=arthurcc

There are hdpi versions of the icons available; I looked at a few other uses of the new Notification API and, while some use the old NotificationHelper getImageURI, many just provide the URI for a single image (whilst other hdpi versions are available alongside).

E.g:
https://github.com/mozilla-b2g/gaia/blob/0a864988f5dce7f9f3dea9609e8ef054679c30ff/apps/bluetooth/js/pair_manager.js#L92
https://github.com/mozilla-b2g/gaia/blob/0a864988f5dce7f9f3dea9609e8ef054679c30ff/apps/system/js/captive_portal.js#L25
https://github.com/mozilla-b2g/gaia/blob/0a864988f5dce7f9f3dea9609e8ef054679c30ff/apps/system/js/icc_worker.js#L508

I'd assumed the notification did automatic hdpi scaling if high-res images are available. Is that not the case?