The aclchecker script must report errors to stderr
    
    Otherwise the error message gets cut and the user does not see it
    and ends up with a weird error instead:
    ``fatal: protocol error: bad line length character: No s``
    (While it should be: `No such project`).
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>