Add error to JsValue conversion and example (#691)

examples/wasm_github_fetch/index.js

@@ -0,0 +1,12 @@
+const rust = import('./pkg');
+
+rust
+    .then(m => {
+        return m.run().then((data) => {
+            console.log(data);
+
+            console.log("The latest commit to the wasm-bindgen %s branch is:", data.name);
+            console.log("%s, authored by %s <%s>", data.commit.sha, data.commit.commit.author.name, data.commit.commit.author.email);
+        })
+    })
+    .catch(console.error);