libafs: log error code when marking server down

Log the error code when marking the server down due
to server/networks issues.

Reviewed-on: http://gerrit.openafs.org/7138
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit f0b039fc4761bb0151c7ee69110926ef16e264c7)

Change-Id: Ib66457a75006858b9523fa44c164859e8c8fae6f
Reviewed-on: http://gerrit.openafs.org/9473
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>