handle missing social accounts for people

* Not all people will have Facebook/Twitter pages.
* Instead of trying to render a non-existing page/tweets, 
* Before this, we tried to render things that didn't exist
* Now we just tell the user that the person they’re interested in has no Facebook or Twitter page, as the case may be.
* This fixes #51 (note that it keeps the tab for a consistent UI, but otherwise handles the situation more gracefully than previously).