Display Posts widget: reorganize before_widget and after_widget

Both args should always be displayed.

In some cases, `before_widget` and `after_widget` were not properly displayed around the Display Posts widget. This commit fixes the problem.

Here is a screenshot with 3 different Display Posts widgets, with 3 different status (error, success, error). I added the words `BEFORE WIDGET` and `AFTER WIDGET` to show where the args were displayed.

* Before the fix:
![screen shot 2014-01-02 at 3 21 46 pm](https://f.cloud.github.com/assets/426388/1832613/dd6aae92-73b9-11e3-86ac-0d3df777075b.png)

* After the fix:
![screen shot 2014-01-02 at 3 21 14 pm](https://f.cloud.github.com/assets/426388/1832615/e6783e78-73b9-11e3-9c84-ebf2e88512b0.png)

Reported here:
* http://wordpress.org/support/topic/wordpress-post-widgetphp-misses-echo-argsafter_widget-invalidating-html?replies=1