Log more detail when IXFR fails with 'failed while receiving responses: not exact'

There are actually two things that it would be helpful to log:

When the zone update fails, what is the exact scenario was that caused the failure (including the RRs involved).

In this specific instance, it was a duplicate add, generated erroneously by a registry application, but not knowing why the zone update failed made the troubleshooting much more difficult.

Leading on from 1, it was discovered that while named will fail to accept an IXFR'd ADD that is a duplicate when this arrives in a new incremental update, it will ignore duplicate ADDs that are part of the same incremental update. This was somewhat of a surprise, and which also contributed to the difficulty troubleshooting (because 'we know there are occasional duplicates, but they don't usually cause a problem').

Therefore, if named is going to silently discard some duplicate adds, it should at least log that it did so!