altos: Complain about sensor self-test errors only in idle mode

altos: Complain about sensor self-test errors only in idle mode

When the accelerometer says to go into pad mode, don't look for other
sensor self test errors. Only look for sensor self test errors to
choose between idle and invalid mode. This will prevent minor sensor
self test errors from letting the rocket fly safely.

Signed-off-by: Keith Packard <keithp@keithp.com>