Solved FF Joint Command cross-commanding.

The feed forward commands being published from the Joint Trajectory
Action Server were being sent to the wrong joints. This was due to
neglecting to reorder the joint angles, before publishing feed forward.
joint_s0 <--> joint_e0 and joint_s1 <--> joint_e1
This crossover prevented the full potential of the feed forward
trajectories from being visible.

Along the way in debugging, I made improvements to increase
debugging verbosity, and the velocity error limits.