Stop iterator when error is encountered, and report.

error_iterator.rs

use anyhow::{anyhow, Result};

fn main() -> Result<()> {
    let v = vec![false, true, true];

    let res = v.iter().map(|x| {
        println!("looped"); // Loops only two times, not 3.
        if *x {
            Err(anyhow!("error"))
        } else {
            Ok(())
        }
    }).collect();

    res
}