Clearer error message if gradient not implement, and formatting.
Clearer error message if gradient not implement, and formatting.

melgor/autograd,kcarnold/autograd,hips/autograd,HIPS/autograd,FighterLYL/autograd,hips/autograd,barak/autograd,HIPS/autograd,jackkamm/autograd,t1m0thy/autograd