Change parse_args readme and samples to catch const std::exception&

[cobyj33]

parse_args can throw exceptions that are not based on std::runtime_error, and the error message should show on all errors.

For example, if a user is using a program formatted as:

parser.add_argument("number")
    .help("Enter a number")
    .scan<'i', int>();

try {
    parser.parse_args(argc, argv);
} catch (const std::runtime_error &e) {
    std::cerr << e.what() << std::endl;
    std::cerr << parser;
    return EXIT_FAILURE;
}

and the user messes up by misformatting the "number" positional argument, this will throw an std::invalid_argument from argparse::details::do_from_chars. However, std::invalid_argument inherits from std::logic_error, not std::runtime_error, so the user will just have an aborted program with no help message. In order to keep the code simple while not having to catch std::runtime_error and std::logic_error separately, the user could use std::exception instead to catch all errors of parse_args.