show a proper usage message when no subcommand is given
    
    This was a change (you might say, regression) in Python 3.3, see:
        https://bugs.python.org/issue9253#msg186387
        http://stackoverflow.com/a/18283730/120202
    
    (cherry picked from commit 8fcc13aa99f83514df0d1528c06de539edb77db3)
    
    Bug: 1445990
    Change-Id: I12774e1675b22025562bb5792b88283c99f75388