Audio Coach should not silently ignore missing statistic

At present if Heart rate is configured within the Audio Coach but not being actually measured, it is silently ignored when each notification occurs. For example if each minute there are statistics for distance and heart rate but for whatever reason there is no heart rate data, the notification will be just "distance <X> kilometres". If the user is not listening, or there is a lot of noise, they may not realize there is an issue, that could otherwise be quickly resolved.

My suggestion for this scenario (and example) would be to say something like "distance <X> kilometres, heart rate error", or "unknown" or anything.

There may be other statistics which are similarly ignored but I haven't tested:

• cadence
• energy
• humidity
• temperature

This idea would be very complementary to https://help.locusmap.eu/topic/38633-give-audio-coach-notification-when-started for finding incorrect configurations.