Added necessary const to recently added function

I just added the ``getEndEffectorTips()`` function a week ago https://github.com/ros-planning/moveit_core/pull/189/files, so this isn't really an API change.

I also snuck in some minor debug output clarifications

For use with https://github.com/ros-planning/moveit_ros/pull/484 and [joystick control functionality](https://github.com/ros-planning/moveit_ros/pull/471)

@garaemon