vol: log error reason on header read failure

vol: log error reason on header read failure

Log the error reason instead of just VSALVAGE when
ReadHeader() fails.

Reviewed-on: http://gerrit.openafs.org/6108
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 0d0a8288c1cdd05bbf5717ac45638cf6760ee7a8)

Change-Id: Ie49c9ee3ea23873f8d71c80fda45b763bcd8e466
Reviewed-on: http://gerrit.openafs.org/6328
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>