Improves server waiting and killing

Makes the client force stop servers if the stop button is clicked more than once
Prevents new servers from starting after server stop is initialized

src/main/java/net/knarcraft/minecraftserverlauncher/userinterface/ServerLauncherGUI.java

@@ -401,6 +401,9 @@ public class ServerLauncherGUI extends MessageHandler implements ActionListener,
         } catch (IOException e1) {
             showError("Could not stop server.");
             e1.printStackTrace();
+        } catch (InterruptedException e) {
+            showError("Could not kill server.");
+            e.printStackTrace();
         }
     }