glusterd: Set errstr appropriately on peer op failure

Change-Id: I27f5f7cd54115d7b236b42f6beaaa05a8b379dd7
BUG: 1010153
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/5978
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>