Show an error if a conversation window fails to init

I saw this happen once. Seems to be a race condition.

js/conversation_panel.js

@@ -22,9 +22,13 @@
     extension.windows.getCurrent(function (windowInfo) {
         var bg = extension.windows.getBackground();
         var conversation = bg.getConversationForWindow(windowInfo.id);
+        if (conversation) {
             window.document.title = conversation.getTitle();
             new Whisper.ConversationView({
                 model: conversation
             }).$el.prependTo($('body'));
+        } else {
+            $('<div>').text('Error').prependTo($('body'));
+        }
     });
 }());