Raise a nicer error message when trying to call gradients with while …

…loop (tensorflow#18052)

* Produce a nicer error message when trying to call gradients on a while loop without properly serializing graph via MetaGraphDef

* Fix syntax and lint error

* Fix minor intent: Wrong continued indentation (add 2 spaces)

tensorflow/python/ops/control_flow_ops.py

@@ -833,6 +833,9 @@ def __init__(self, forward_ctxt, outer_grad_state):
     if outer_grad_state:
       outer_forward_ctxt = outer_grad_state.forward_context
     else:
+      if not hasattr(forward_ctxt, 'outer_context'):
+        raise ValueError("Failed to call gradients on a while loop without"
+                         "properly serializing graph via MetaGraphDef")
       outer_forward_ctxt = forward_ctxt.outer_context
 
     # Add the forward loop counter.