Remove downloaded invalid file
    
    By using pycurl to download a file from lookaside, invalid content sent
    back from server is stored into the downloaded file, even if server does
    not response such as 404. The downloaded file should be removed in order
    to avoid many confusion to packagers.
    
    Fix #79
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>