axfr/ixfr/notify: check RCODE, better log messages

src/common/errcode.c

src/common/errcode.h

src/knot/nameserver/axfr.c

src/knot/nameserver/axfr.h

src/knot/nameserver/ixfr.c

src/knot/nameserver/ixfr.h

src/knot/nameserver/notify.c

src/knot/nameserver/requestor.c