Various bug fixes

- Fixed bug where an invalid port number would crash the program
- Closed the stage before cleaning up resources. This speeds up closing the app.
- Added error handling for when the client looses connection to the server.

Tags: #story[1281]

src/main/java/seng302/visualiser/controllers/ViewManager.java

@@ -272,6 +272,8 @@ public class ViewManager {
     }
 
     private void closeAll() {
+        if (stage!= null) stage.close();
+
         try {
             ServerAdvertiser.getInstance().unregister();
         } catch (IOException e1) {