update error messages.
update error messages.

keulYSMB/urdfdom,shadow-robot/urdfdom,robotology-dependencies/urdfdom,keulYSMB/urdfdom,robotology-dependencies/urdfdom,robotology-dependencies/urdfdom,shadow-robot/urdfdom,keulYSMB/urdfdom,robotology-dependencies/urdfdom,shadow-robot/urdfdom,keulYSMB/urdfdom,shadow-robot/urdfdom