Forbid replacing multi-output nodes directly until we have a clear valid use case in mind (#745)

* making replace_node more robust

* fix error msg

* tweak err msg

src/ngraph/graph_util.cpp

@@ -123,6 +123,13 @@ void ngraph::replace_node(std::shared_ptr<Node> target, std::shared_ptr<Node> re
         throw ngraph_error("Result nodes cannot be replaced.");
     }
 
+    if (target->get_outputs().size() > 1)
+    {
+        throw ngraph_error(
+            "Multi-output nodes should not be replaced directly. The corresponding "
+            "GetOutputElements should be replaced instead");
+    }
+
     // Fix input/output descriptors
     assert(target->get_outputs().size() == replacement->get_outputs().size());