"Bad battery" message is misleading and unnecessarily scary

With a Copter running 4.4 (and perhaps other versions) MP displays (and says) "Bad Battery" when the battery voltage is low. This is likely because AP is setting the [SENSOR_BATTERY](https://github.com/ArduPilot/mavlink/blob/master/message_definitions/v1.0/common.xml#L199) bit in the [SYS_STATUS message](https://github.com/ArduPilot/mavlink/blob/master/message_definitions/v1.0/common.xml#L4128C8-L4128C8).

I believe we have a conflict with MAVProxy which uses this bit to change the colour of the battery label (or icon) to red but MP interprets this as a battery failure.

It's really not a failure though at least in the way that the AP flight code interprets it. So MP voicing "Bad Battery" is just unnecessarily scary.