background: Don't silently fizzle out when removing bad content from the cache

If the background is already removed, or we're trying to remove bad content,
this is probably a bug in content accounting, so let us crash so we can fix
the bugs.

https://bugzilla.gnome.org/show_bug.cgi?id=719803

js/ui/background.js

@@ -113,8 +113,8 @@ const BackgroundCache = new Lang.Class({
 
     _removeContent: function(contentList, content) {
         let index = contentList.indexOf(content);
-
-        if (index >= 0)
+        if (index < 0)
+            throw new Error("Trying to remove invalid content: " + content);
         contentList.splice(index, 1);
     },