Detect soft joint limits set to 0, show warning or error.

[hersh]

Playing with the motoman model, I was frustrated that I could not get any joints to move. Turns out it was the <safety_controller> tags which all had soft_lower_limit and soft_upper_limit set to 0. I'm not sure why the lines were in there at all, but it seems like an easy thing to detect in the setup assistant: if a joint cannot move because of the joint limits, show a warning message saying why the joint can't move.