Make run_mirror more verbose about failures.

When run_mirror fails to run pip install or downloads print the project
and branch that failed to make debugging problems easier.

Change-Id: I2fec996005010de0e1c6d72b026e09a1e1143acb
Reviewed-on: https://review.openstack.org/23614
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins