Improve melter error message

pdxu-app/src/main/java/com/crschnick/pdxu/app/util/integration/RakalyHelper.java

@@ -4,6 +4,7 @@
 import com.crschnick.pdxu.app.savegame.SavegameContext;
 
 import java.io.IOException;
+import java.nio.file.Files;
 import java.nio.file.Path;
 
 public class RakalyHelper {
@@ -15,6 +16,7 @@ public static boolean shouldShowButton(SavegameContext<?, ?> context) {
 
     public static byte[] toMeltedPlaintext(Path file) throws Exception {
         var melter = PdxuInstallation.getInstance().getRakalyExecutable();
+        check(melter);
         var proc = new ProcessBuilder(
                 melter.toString(),
                 "melt",
@@ -35,8 +37,10 @@ public static byte[] toMeltedPlaintext(Path file) throws Exception {
     }
 
     public static byte[] toEquivalentPlaintext(Path file) throws Exception {
+        var melter = PdxuInstallation.getInstance().getRakalyExecutable();
+        check(melter);
         var proc = new ProcessBuilder(
-                PdxuInstallation.getInstance().getRakalyExecutable().toString(),
+                melter.toString(),
                 "melt",
                 "--unknown-key", "stringify",
                 "--retain",
@@ -54,4 +58,10 @@ public static byte[] toEquivalentPlaintext(Path file) throws Exception {
 
         return b;
     }
+
+    private static void check(Path file) throws IOException {
+        if (!Files.exists(file)) {
+            throw new IOException("Ironman melter executable " + file + " is missing. This is usually caused by Windows Defender or another AntiVirus program removing the melter executable because it thinks it's malicious.\n\nYou can try deleting %LOCALAPPDATA%\\Programs\\Pdx-Unlimiter\\app\\ and relaunching pdxu. That should trigger a new download. If the file then gets removed again, you probably have to add an exception to your antivirus.");
+        }
+    }
 }