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.

Change-Id: Iad3c2c857f1b736fcf4a142338d9e219bab68969
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>