Make URDF parser stricter about required attributes.
Make URDF parser stricter about required attributes.

Following the spec at http://wiki.ros.org/urdf/XML

PiperOrigin-RevId: 470192022
Change-Id: I4699b6f87fff5706b3ba5f33bb8242bb2ad33478

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco