Re-throw exceptions immediately for completed generators.

This happens to fix #210 indirectly, because it causes statements like
`return Promise.reject(error)` to throw, but there's a better way of
fixing that bug.

Author: benjamn

runtime.js

@@ -207,6 +207,10 @@
       }
 
       if (state === GenStateCompleted) {
+        if (method === "throw") {
+          throw arg;
+        }
+
         // Be forgiving, per 25.3.3.3.3 of the spec:
         // https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generatorresume
         return doneResult();