added builtin.sum message to TensorResult.__iter__

tensor.py

@@ -390,7 +390,8 @@ class _tensor_py_operators:
 
     def __iter__(self): 
         # This prevents accidental iteration via builtin.sum(self)
-        raise TypeError('Tensor does not support iteration')
+        raise TypeError('Tensor does not support iteration. '
+        'Maybe you are using builtin.sum instead of theano.tensor.sum?')