ref #3911 - gives preferred status to first mapper errors

[adagios]

Implements option 2.1 discussed on the Issue: logs errors for all mappers, giving more importance to the preferred one.

I wrote a test to confirm the behaviour, but it produces a lot of error logging and I don't know if it makes sense to include it. I tend to dislike tests that log a lot of stacktraces.

[frantuma]

Thanks!