usermanager CLI: more cleanup

usermanager CLI: use success messages

This makes it more consistent with the other tools

usermanager CLI: fix parameter order for creating users

usermanager CLI: catch plugin signalled error messages

Plugins may prevent user modifications and will use msg() to tell why.
This will catch the messages and print them on CLI as well

abort early when no auth backend is available

type hints and cleanup

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typos

remove unused variable

add addToGroup and removeFromGroup

initial usermanager cli

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