Add default impl of AsyncActionNode#on_halted

Author: kylc

btpy/__init__.py

@@ -64,3 +64,7 @@ def on_running(self) -> NodeStatus:
             # Otherwise, just assume the action finished successfully.
             else:
                 return NodeStatus.SUCCESS
+
+    def on_halted(self):
+        # Default action: do nothing
+        pass