Move some checks from cli.py to luxi.py

The idea of cli.py and luxi.py is that all protocol checks should be in
luxi, and cli.py should just offer some helpful shortcuts for the
command line scripts.

This patch removes the result checks from cli and adds some other checks
to luxi. It does no longer check the success/failure since it's not yet
clear how that should be handled - probably exceptions.

Reviewed-by: ultrotter