Error display is relatively blurry and not precise enough

[baoyachi]

• the code

  println!("{:?}",parse("0mxx"));

• the output

  Err(DSLError("0mxx\n ^\n"))

[baoyachi]

• current not support below error

//TODO lost cause, need fix
        let err = format!("{}", parse("3ms-2ms").err().unwrap());
        assert_eq!(err, r#"
3ms-2ms
   ^
partial_input:`-2ms`,error Eof"#.trim());

[baoyachi]

now, support precise error reporting.

use duration_str::parse;

fn main() {
    let err = format!("{}", parse("3ms-2ms").err().unwrap());
    assert_eq!(err, r#"
3ms-2ms
   ^
partial_input:-2ms, expect one of:["+", "*"], but find:-2ms"#.trim());
}