Getting a line slice requires two error checks

[jyn514]

Presumably line_span always returns a valid span. However, the API requires you to check the error anyway:

    let line = file_db
        .line_span(file, start.line)
        .expect("line should be in bounds");
    println!("{}", file_db.source_slice(file, line).expect("line is still in bounds"));

I would like Files to get a new line_slice API which returns a Result<&str, LineIndexOutOfBoundsError> directly.