Bug 1097835 - [Camera] App doesn't listen for closed events

Would it make sense to reuse the `camera-unavailable-*` strings for this too? I noticed that this bug is marked 2.1+, and we had to struggle to get the `camera-unavailable-*` strings in late. I'm not even 100% sure if they'll be localized in time for v2.1, so adding more new strings is probably not possible at the time (at least for v2.1).