Fix link access in ik fail log and add test code

(irtmodel.l, test-irt-motion.l) : Fix link access in ik fail log and add test code for it.
This bug is reported in https://github.com/jsk-ros-pkg/jsk_roseus/issues/139

Use ``:link`` for link access.
The accessor specification was discussed in 
https://github.com/jsk-ros-pkg/jsk_model_tools/issues/18#issuecomment-44769851