Add console.log(err) to see errors from eval()

If there is any problem with components, the code that is in use now
`React.render(<Alert ...)` is not enough.

Upgrading to `babel@5` has been brought it to light.

docs/src/ReactPlayground.js

@@ -250,6 +250,7 @@ const ReactPlayground = React.createClass({
         /* eslint-enable */
       }
     } catch (err) {
+      console.log(err);
       this.setTimeout(() => {
         React.render(
           <Alert bsStyle='danger'>{err.toString()}</Alert>,