'Warn the user that a given chunk of code is not valid Python 3,
but that it cannot be converted automatically.
First argument is the top-level node for the code in question.
Optional second argument is why it can\'t be converted.'

def cannot_convert(self, node, reason=None):

    lineno = node.get_lineno()
    for_output = node.clone()
    for_output.prefix = u''
    msg = 'Line    %d:    could    not    convert:    %s'
    self.log_message((msg % (lineno, for_output)))
    if reason:
        self.log_message(reason)
