Check availability of devices before executing a command

[gcampax]

If a device is not available (e.g. it is a physical device and it is off or disconnected, or the authentication is no longer valid), we should inform the user nicely and maybe prompt the user to configure the device again (= refresh the access token), instead of trying the command and failing miserably.

This is something I've meant to do for a long time, and it is also the reason why devices have a checkAvailable method. I'm opening this issue so we don't forget again...