Missing commas produce unhelpful errors with col/line 0

[WaffleLapkin]

This code:

    #[derive(serde::Deserialize)]
    struct Test {
        a: i32,
        b: i32,
    }
    let string = r#"Test(
        a: 1 // <-- forgetten comma here
        b: 2
    )"#;

    ron::from_str::<Test>(string).unwrap();

Produces the following error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { code: Message("missing field `b`"), position: Position { col: 0, line: 0 } }', src/main.rs:67:35

Which is really hard to understand because

1. it says field b is missing but it doesn't
2. it doesn't say anything about commas
3. it points to 0, 0

[kvark]

On a more general note, I was wondering if we could integrate https://github.com/brendanzab/codespan for proper error reporting.