Lets render return a promise for doing async work

This allows userland code to perform async actions and render the HTML
some time in the future. This opens up the possibility for doing async
rendering, data fetching prior to rendering, code splitting with
routing, etc.

src/utils/BatchManager.js

@@ -149,22 +149,18 @@ class BatchManager {
       if (!renderFn || typeof renderFn !== 'function') {
         // component not registered
         context.statusCode = 404;
-        context.duration = msSince(start);
-        throw notFound(name);
+        return Promise.reject(notFound(name));
       }
 
-      let response = null;
-
-      // render the component!
-      try {
-        context.html = renderFn(context.props);
-      } catch (e) {
-        response = Promise.reject(e);
-      } finally {
-        context.duration = msSince(start);
-      }
-
-      return response;
+      return Promise.resolve(renderFn(context.props));
+    })
+    .then((html) => {
+      context.html = html;
+      context.duration = msSince(start);
+    })
+    .catch((err) => {
+      context.duration = msSince(start);
+      return Promise.reject(err);
     });
   }