commander : hotplug sensor support, better failure reporting

msg/vehicle_status.msg

src/modules/commander/PreflightCheck.cpp

src/modules/commander/PreflightCheck.h

src/modules/commander/commander.cpp

src/modules/commander/state_machine_helper.cpp

src/modules/commander/state_machine_helper.h

src/modules/systemlib/rc_check.c

src/modules/systemlib/rc_check.h