`std::run` functions fail after io_error

[klutzy]

use std::io;
use std::run;

fn main() {
    io::io_error::cond.trap(|_| {}).inside({
        || {
            let opts = run::ProcessOptions::new();
            let _ = run::Process::new("", [], opts); // io_error
        }
    })
}

Result:

task '<main>' failed at 'called `Option::unwrap()` on a `None` value', C:\home\stone\rust-vanilla\src\libstd\option.rs:135

Similarly std::run::{process_status, process_output} fails if it fails to make new process.

Previously #10654 suggested a potential fix using Result but the patch would sound after #10449 lands.