Merge branch 'zone_loading_failed_diff' into 'master'

Zone Loading Failed Diff + test cases

See the commit messages, should be clear.
Also, the `failed to sign the zone:` message appears also when ixfr-from-differences fails, because it's together in one function (so it prints out even if I don't have DNSSEC signing enabled).
I'm not sure how to handle this properly, maybe have differences and zone signing in a separate functions and print the reason from within.