Validate provided arguments and act on --help.

def validate_args(args):
    """Validate provided arguments and act on --help."""
    # Check correctness of similarity dataset names
    for dataset_name in args.similarity_datasets:
        if dataset_name.lower() not in map(
                str.lower,
                nlp.data.word_embedding_evaluation.word_similarity_datasets):
            print('{} is not a supported dataset.'.format(dataset_name))
            sys.exit(1)

    # Check correctness of analogy dataset names
    for dataset_name in args.analogy_datasets:
        if dataset_name.lower() not in map(
                str.lower,
                nlp.data.word_embedding_evaluation.word_analogy_datasets):
            print('{} is not a supported dataset.'.format(dataset_name))
            sys.exit(1)