Same as shell_out but doesn't raise if the cmd exits badly.

def shell_out_ignore_exitcode(cmd, stderr=STDOUT, cwd=None):
    """Same as shell_out but doesn't raise if the cmd exits badly."""
    try:
        return shell_out(cmd, stderr=stderr, cwd=cwd)
    except CalledProcessError as c:
        return _clean_output(c.output)