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

You may want to add a layer of indirection in here. I'm adding a 'reason' string to onClosed in bug 1099390, and simplifying some of the Gecko code so that you'll get a callback as a normal part of calling the release() method.

Reasons are: "SystemFailure", "HardwareReleased", and "NotAvailable" -- though the last value isn't emitted yet.