I've installed http://downloads.linux.hp.com/SDR/downloads/MCP/ubuntu/pool/non-free/hpacucli_9.40.1-1._amd64.deb on our new servers, they have P420i with "Firmware Version: 5.42".
I don't get the warning you are seeing when I run any of the raid commands.
Maybe you need to update the raid firmware? (contrary to the error of course!)
