Add exceptions for unsupported actions.
Remove channels from Root.
Raise NotSupportedByBra by default on request handler methods.
Catch NotSupported errors and describe them in the app.
Show response text rather than message.
Add handlers to items added to Players.

Currently the handler doesn't do much, because there does not appear
to be a way to unload items in BAPS, but this solves the issue of
Items having no handler whatsoever.