VDSM: Fix VDSM error message.

Rephrase VDSM error message.

Change-Id: I8ee2f8c9bbeecb589267b7fdaca8442210debfbb
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/9355
Reviewed-by: Igor Lvovsky <ilvovsky@redhat.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Daniel Erez <derez@redhat.com>
Reviewed-by: Liron Aravot <laravot@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>