volume: Log the correct error when creating a volume fails

When a volume creation failed because of CannotCreateLogicalVolume
exception, we used to lie and log "volume already exists". This log
confused and wasted many developers hours. Now we log the exception
value instead.

Change-Id: I603b055658950dae5ccc3806b8b7a9e53762c5ef
Bug-Url: https://bugzilla.redhat.com/1143830
Relates-To: https://bugzilla.redhat.com/1142710
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33301
Reviewed-by: Yoav Kleinberger <ykleinbe@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>