Comment 2 for bug 1058072

Edward, thanks for that work, and I'm sorry I didn't see it at the time (I wasn't subscribed to software-properties). Is it possible to distinguish between the case of clicking Cancel, and the case where PolicyKit fails for any other reason? I think we want to show an error alert only in the latter case. And perhaps make it a RecoverableError <https://wiki.ubuntu.com/ErrorTracker#app-requested>, so we can figure out what's causing it, and fix root causes later on.