Graph.Operation: Throw ValueError exception when `Current graph is not default graph.`

src/TensorFlowNET.Core/Graphs/Graph.Operation.cs

@@ -54,19 +54,21 @@ namespace Tensorflow
                     var handle = return_oper_handle.node + tf_op_size * i;
                     return_opers[i] = new Operation(*(IntPtr*)handle);
                 }
-            }
-
+            }
+
             return return_opers;
         }
 
         public Operation OperationByName(string operName)
         {
             var handle = c_api.TF_GraphOperationByName(_handle, operName);
-            if(graph_key != tf.get_default_graph().graph_key)
-            {
-                Console.WriteLine($"Current graph is not default graph.");
-                // throw new ValueError($"Current graph is not default graph.");
+            var defaultKey = tf.get_default_graph().graph_key;
+            if (graph_key != defaultKey)
+            {
+                //Console.WriteLine($"Current graph is not default graph.");
+                throw new ValueError($"Current graph is not default graph. Default Graph Key: {defaultKey}, Current Graph Key: {graph_key}");
             }
+
             return new Operation(handle, g: this);
         }