volser: correctly attribute 'vos partinfo' errors

volser: correctly attribute 'vos partinfo' errors

Since the original IBM code import, the 'vos partinfo' error message
blames the wrong command, 'vos listpart':

    $ vos partinfo afs01.sinenomine.net
    <unknown>: Could not get afs tokens, running unauthenticated.
    Could not fetch the list of partitions from the server
    Possible communication failure
    Error in vos listpart command.
    Possible communication failure

Correct the error message to specify 'vos partinfo' instead.

Change-Id: I966dee8c679db89c7ed5ce21d31ebc7424803cf2
Reviewed-on: https://gerrit.openafs.org/14489
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>