make: log error to stderr
make: log error to stderr

In order to not pollute the output given by
flavors-modified-vs-master.py we log the errors on stderr and not
stdout.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-docker,JrCs/ceph-docker,JrCs/ceph-docker,ceph/ceph-docker,JrCs/ceph-docker