'See if the tarball aleady exists.'

def check_for_image_tarball(self):

    uri = self.options.tarball_uri
    if (not uri.startswith('gs://')):
        error = '--tarball_uri    must    be    a    Google    Cloud    Storage    URI,    not    "{uri}"'.format(uri=uri)
        raise ValueError(error)
    result = run_quick('gsutil    ls    {uri}'.format(uri=uri), echo=False)
    if (not result.returncode):
        error = 'tarball    "{uri}"    already    exists.'.format(uri=uri)
        raise ValueError(error)
