Does some much needed cleanup, adds the Paper server type and fixes broken downloads

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

@@ -0,0 +1,61 @@
+package net.knarcraft.minecraftserverlauncher.userinterface;
+
+import javax.swing.*;
+
+public abstract class MessageHandler implements GUI {
+
+    private final boolean silent;
+
+    /***
+     * Initializes a new message handler
+     *
+     * @param silent <p>Whether to print to cli instead of showing a GUI element</p>
+     */
+    public MessageHandler(boolean silent) {
+        this.silent = silent;
+    }
+
+    @Override
+    public void showError(String title, String message) {
+        if (silent) {
+            System.out.println(message);
+        } else {
+            showJOptionPane(title, message, JOptionPane.ERROR_MESSAGE);
+        }
+    }
+
+    @Override
+    public void showError(String message) {
+        showError("Error", message);
+    }
+
+    @Override
+    public void showMessage(String title, String message) {
+        if (silent) {
+            System.out.println(message);
+        } else {
+            showJOptionPane(title, message, JOptionPane.INFORMATION_MESSAGE);
+        }
+    }
+
+    @Override
+    public void showMessage(String message) {
+        showMessage("Info", message);
+    }
+
+    /**
+     * Shows a JOptionPane
+     *
+     * @param title <p>The title of the pane</p>
+     * @param message <p>The message of the pane</p>
+     * @param paneType <p>The type of the pane</p>
+     */
+    private void showJOptionPane(String title, String message, int paneType) {
+        JOptionPane.showMessageDialog(
+                null,
+                message,
+                title,
+                paneType
+        );
+    }
+}