win: FormatMessage failure when io failed

[klutzy]

use std::io;

fn main() {
    let output = Path::new("badpath\\a.txt"); // badpath doesn't exist
    let _ret = io::File::create(&output); // so this is expected to return Err
}

task '<main>' failed at '[15100] FormatMessage failure', C:\home\stone\rust-vanilla\src\libstd\os.rs:743

Reproduced on rust revision 092afdb, windows 8.1 korean locale.

cc #13073