native: add more info to the native unimplemented error.

This refers to green, which (AFAICT) has everything implemented. In
particular, this will help guide people to get working signal handling
via libgreen.

src/libnative/io/mod.rs

@@ -77,7 +77,7 @@ fn unimpl() -> IoError {
     IoError {
         code: ERROR as uint,
         extra: 0,
-        detail: None,
+        detail: Some("not yet supported by the `native` runtime, maybe try `green`.".to_string()),
     }
 }