olpc_battery: Add an 'error' sysfs device that displays raw errors

Grab the error code from EC_BAT_ERRCODE and let the user see it (rather
than attempting to decode it as we do with PROP_HEALTH) with a separate
error sysfs file.

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>