Pass errors to front end

[rikhuijzer]

It should be made more easy to pass errors to the front end. This came up during #17, where it can throw an error:

"withinRepeatedANOVA" => {
    let k = parse_i64(data, "k").unwrap();
    let m = parse_i64(data, "m").unwrap();
    let rho = parse_f64(data, "rho").unwrap();
    let epsilon = parse_f64(data, "epsilon").unwrap();
    if epsilon < (1.0 / (m as f64 - 1.0)) {
        Err(
            "lower bound of ε corresponds to 1 / (number of measurements - 1)"
                .to_string(),
        )