Invalid change_node_key response for remote node

[PawelPodkalicki]

If function change_node_key is called with node parameter different than user node, response is invalid. When node exists reponse has result field with text Node key changed.

If node doesn't exist error message is Setting key for remote bank failed, which is correct behaviour.

[jzemlo]

Setting remote node key is a valid operation only on a node that is in double-spent state. Client should check for this condition before accepting transaction.