fix: Add check for invalid modern forwarding config

src/main/java/me/joezwet/fabricforwarding/FabricForwarding.java

@@ -19,7 +19,8 @@ public class FabricForwarding implements DedicatedServerModInitializer {
     @Override
     public void onInitializeServer() {
         if(Config.getInstance().getMode().equals(ForwardingMode.MODERN) && Config.getInstance().getSecret().length == 0) {
-            LOGGER.fatal("Modern IP forwarding enabled but no secret provided, please enter the same secret as the one in your proxy's config.");
+            LOGGER.fatal("Modern IP forwarding enabled but no secret provided, please enter the same secret as the one in your proxy's config. Disabling forwarding.");
+            Config.getInstance().invalidMode();
         }
     }
 }

src/main/java/me/joezwet/fabricforwarding/api/config/Config.java

@@ -13,4 +13,6 @@ static Config getInstance() {
 
     ForwardingMode getMode();
     byte[] getSecret();
+
+    void invalidMode();
 }

src/main/java/me/joezwet/fabricforwarding/config/ConfigImpl.java

@@ -26,4 +26,9 @@ public ForwardingMode getMode() {
     public byte[] getSecret() {
         return this.secret.getBytes(Charsets.UTF_8);
     }
+
+    @Override
+    public void invalidMode() {
+        this.mode = "OFF";
+    }
 }