cli : Display error messages if virt file has been deleted or is invalid.

"gluster volume set <VOLNAME> group virt" will display error message
if virt file is deleted or is invalid.

Change-Id: Icb202b6a445597fcd9a3dcef8001891f2601a115
BUG: 916127
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4586
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>