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

src/main/java/net/knarcraft/minecraftserverlauncher/utility/JarDownloader.java

@@ -0,0 +1,68 @@
+package net.knarcraft.minecraftserverlauncher.utility;
+
+import net.knarcraft.minecraftserverlauncher.server.servertypes.ServerType;
+import net.knarcraft.minecraftserverlauncher.server.ServerTypeHandler;
+import net.knarcraft.minecraftserverlauncher.userinterface.GUI;
+
+import javax.naming.ConfigurationException;
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+
+/**
+ * This class handles all downloading of .jar files
+ */
+public class JarDownloader {
+
+    private final String jarDirectory;
+    private GUI gui;
+
+    /**
+     * Initializes a jar downloader
+     *
+     * @param gui          <p>The GUI to use for displaying messages</p>
+     * @param jarDirectory <p>The directory to download jar files to</p>
+     */
+    public JarDownloader(GUI gui, String jarDirectory) {
+        this.gui = gui;
+        this.jarDirectory = jarDirectory;
+    }
+
+    /**
+     * Downloads all jars to the program directory.
+     *
+     * @throws IOException On version file failure or folder creation failure
+     */
+    public void downloadJars() throws IOException {
+        if (!new File(jarDirectory).exists() && !new File(jarDirectory).mkdirs()) {
+            gui.showError("Could not create the Jars folder. Please run the program with admin permissions, or move it to " +
+                    "a writable directory.");
+            throw new FileNotFoundException("Unable to create jars folder");
+        }
+        try {
+            downloadAll();
+            gui.printMessageToGUI("Finished downloading jars");
+        } catch (FileNotFoundException | ConfigurationException e) {
+            throw new FileNotFoundException("One or more downloads failed: " + e.getMessage());
+        }
+    }
+
+    /**
+     * Downloads jar files for all possible server versions
+     *
+     * @throws IOException <p>If a jar fails to download</p>
+     */
+    private void downloadAll() throws IOException, ConfigurationException {
+        for (ServerType type : ServerTypeHandler.getServerTypes()) {
+            if (type.getName().equals("Custom")) {
+                continue;
+            }
+            for (String version : type.getVersions()) {
+                boolean success = type.downloadJar(jarDirectory, version);
+                if (!success) {
+                    throw new FileNotFoundException("Unable to download the jar file for " + type.getName() + version);
+                }
+            }
+        }
+    }
+}