afs: Avoid SetupVolume panic

Currently SetupVolume panics if it cannot successfully read a
volumeinfo entry from disk. Try to return an error instead, so we
don't panic the machine.

Reviewed-on: http://gerrit.openafs.org/9094
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 6f7ae535bbac2a5376358801b7f2c9e072f2d141)

Change-Id: Ib8ea06192bfcd6c2111444db325abc4a90190bbc
Reviewed-on: http://gerrit.openafs.org/9131
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>