Ensure examples behave on default backend and/or with no local Bt device

This patch fixes crashes and ensures that examples give some form of
indication why they are not working.

Change-Id: Id99951e8729c0715505e5fff7448a475979a6406
Reviewed-by: Nedim Hadzic <nhadzic@blackberry.com>
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>