Supervised and unlinked task failure modes do not work on Windows

[fmartini]

fn main() {
    let res: Result<(), ()> = do task::try { fail; };
    io::println(res.is_err().to_str());
}

Result on Windows 7:

rust: task failed at 'explicit failure', test.rs:2
rust: domain main @0x2fe450 root task failed

Result on Ubuntu 12.04:

rust: task failed at 'explicit failure', test.rs:2
true

Tasks spawned with spawn_supervisedand spawn_unlinked always kill the root task on Windows when the task fails.

[brson]

This is a symptom of #908. Failure on windows does not work at all.