auto merge of #13115 : huonw/rust/rand-errors, r=alexcrichton

move errno -> IoError converter into std, bubble up OSRng errors

Also adds a general errno -> `~str` converter to `std::os`, and makes the failure messages for the things using `OSRng` (e.g. (transitively) the task-local RNG, meaning hashmap initialisation failures aren't such a black box).

src/test/bench/noise.rs

@@ -41,7 +41,7 @@ struct Noise2DContext {
 
 impl Noise2DContext {
     fn new() -> Noise2DContext {
-        let mut rng = StdRng::new();
+        let mut rng = StdRng::new().unwrap();
 
         let mut rgradients = [Vec2 { x: 0.0, y: 0.0 }, ..256];
         for x in rgradients.mut_iter() {