Better error message for --decoder-attention

        if isinstance(attention, bool):

            # expand True into [True, True, ...] and do the same with False

            attention = [attention] * len(convolutions)

        if not isinstance(attention, list) or len(attention) != len(convolutions):

            raise ValueError('Attention is expected to be a list of booleans of '

                             'length equal to the number of layers.')

        num_embeddings = len(dictionary)

        padding_idx = dictionary.pad()