Cosmetic fixes for mirror script stderr.

* jeepyb/cmd/run_mirror.py: Add newlines to a couple write calls
which were lacking, and also in places where an extra blank line
could provide useful visual separation.

Change-Id: I54e38512717864f02bdc21a577498c0422bf5251
Reviewed-on: https://review.openstack.org/24015
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins