unwraps lead to panic

[JosiahParry]

cron-descriptor-rust/src/description_builder.rs

Line 227 in eb586c0

let mut day_of_week_num = exp.parse::<u8>().unwrap();

let c = get_description_cron("5 0 * * */"); leads to a panic due to the above. I think perhaps rather than infallibly returning String the methods can return Result<String> so that ? can bubble up errors rather than panic.