Z-Wave: Node Deletion & Add Controller IsFailedNode

A Z-Wave node can only be deleted if its present in the controllers
"Failed List" resident in its own NVRAM.
Added the Z-Wave IsFailedNode Message Class.
Added new Failed status if the controller has it flagged as failed in
its own database.
This then allows a node to be deleted.
Visibility of the "Delete Node" button uses this new logic.
I think this block probably ought to be removed? Just call the removefailednode, and let the serialise be done after the event notification is received (you already have the code there as well)?