improve error message for grad of ProdWithoutZeros

theano/tensor/elemwise.py

@@ -2139,3 +2139,8 @@ class ProdWithoutZeros(CAReduceDtype):
     def __init__(self, axis=None, dtype=None, acc_dtype=None):
         CAReduceDtype.__init__(self, mul_without_zeros, axis=axis,
                                dtype=dtype, acc_dtype=acc_dtype)
+    def grad(self, inp, grads):
+        raise NotImplementedError(
+                '2nd derivatives of `product(a)` is not currently supported.' 
+                'If `a` is guarenteed to contains no zeros, use `product(a, no_zeros_in_input=True)`.'
+                )