Joystick: add notice if trying to command motion while disarmed

Particularly useful for beginners, but also for anyone who has accidentally/unknowingly disarmed their vehicle, and are confused as to why it's not moving per their commands. Not critical (since they should be able to figure it out), but would make for a friendlier experience.

The issue names the Joystick interaction but no file or test. Start by locating the Joystick entry point and its handling of the vehicle's armed state; verify that an attempted motion command while disarmed produces a clear notice without changing the requested behavior.

Written by the indexing model from the issue text.