reset: Improve error message when command is missing
    
    If some options were specified without any positional arguments, this
    confusing error message was displayed:
    
        $ sanlk-reset -i 42
        lockspace_name is required
    
    Now more correct message is displayed:
    
        command is required
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>