add debug info

theano/tensor/elemwise.py

@@ -741,7 +741,7 @@ class Elemwise(Op):
             scalar_ograds = map(as_scalar, ograds)
             scalar_igrads = self.scalar_op.grad(scalar_inputs, scalar_ograds)
             for igrad in scalar_igrads:
-                assert igrad is not None
+                assert igrad is not None, self.scalar_op
 
         finally: