AP_Arming tacks on the sub-system bit.
Remove PiccoloCAN's silly nullptr check
Require the library to supply the failure message (no default message)
Remove default cases so authors know to think about places they should
add things.
- When disarmed, broadcast a "software disable" command to all ESC on the bus
- This means that ESC will reject any PWM commands
- When armed, ensure each ESC is enabled before sending PWM commands