How to change the "unknown command" error message with ProtocolLib.

ChangingUnknownCommand.java

package com.comphenix.example;

import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.event.Listener;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketEvent;

public class ChangingUnknownCommand extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        ProtocolLibrary.getProtocolManager().addPacketListener(
                new PacketAdapter(this, ConnectionSide.SERVER_SIDE, Packets.Server.CHAT) {
                    @Override
                    public void onPacketSending(PacketEvent event) {
                        String message = event.getPacket().getStrings().read(0);

                        // Modify this exact message regardless of the coloring
                        if ("Unknown command. Type \"help\" for help.".equals(ChatColor
                                .stripColor(message))) {
                            event.getPacket().getStrings()
                                    .write(0, "Wrong. Try again!");
                        }
                    }
                });
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        sender.sendMessage(ChatColor.RED + "You don't have permission for this area.");
        return true;
    }
}

Note that this only works in Minecraft 1.5.2 and earlier. Use the following version for 1.6.0+:
https://gist.github.com/aadnk/6940010