如何在 Discord.js 中添加错误消息?

I Create a Truth and dare Bot. My Prefix is + Now I want to add an error message to it. I have two variables "t" "d" If anyone types +something which does not match my variable to send a Message that "Invalid Command +help for Help" Can you guys help me?

const Discord = require('discord.js');
const client = new Discord.Client();
const keepAlive = require("./server")
const prefix = "+";

// ======== Ready Log ========
client.on ("ready", () => {
    
    console.log('The Bot Is Ready!');
    client.user.setPresence({
      status: 'ONLINE', // Can Be ONLINE, DND, IDLE, INVISBLE
      activity: {
          name: 'Truth Or Dare | +help',
          type: 'PLAYING', // Can Be WHATCHING, LISTENING
      }
  })
  }); 
// ======== Code ========

client.on('message', message => {
const help = new Discord.MessageEmbed()
    .setColor('#72dfa3')
    .setTitle(`Truth Or Dare`)
    .addFields(
        { name: '``+help``', value: 'For help'},
    { name: '``+t``', value: 'For Truth'},
    { name: '``+d``', value: 'For Your Dare'},
    { name: '``Created By``', value: 'AlpHa Coder [Labib Khan]'},
    )
    .setTimestamp()
  .setFooter(`${message.author.username}`, message.author.displayAvatarURL());
  if (message.content === `${prefix}help`) {
    message.channel.send(help);
  }
});

// ========= Truth =========
client.on('message', message => {
const t = [
"If you could be invisible, what is the first thing you would do?", 
"What's the strangest dream you've ever had?",
"What are the top three things you look for in a boyfriend/girlfriend?",
"What is your worst habit?",
"How many stuffed animals do you own?",
"What is your biggest insecurity?"
]
const truth = t[Math.floor(Math.random() * t.length)];
if (message.content === `${prefix}t`) {
  message.channel.send(truth);
}
});

// ========= Dare =========
client.on('message', message => {
  const d = [
"Do a free-style rap for the next minute.",
"Let another person post a status on your behalf.",
"Hand over your phone to another player who can send a single text saying anything they want to anyone they want.",
"Let the other players go through your phone for one minute.",
"Smell another player's armpit",
"Smell another player's barefoot.",
"Tell everyone your honest opinion of the person who sent this command."
  ]
  const dare = d[Math.floor(Math.random() * d.length)];
  if (message.content === `${prefix}d`) {
    message.channel.send(dare);
  }
});


const token = process.env.TOKEN;
keepAlive()
client.login(token);

请解释清楚，以便我理解.提前谢谢

// Array of possible truth replies
const t = [
    "If you could be invisible, what is the first thing you would do?", 
    "What's the strangest dream you've ever had?",
    "What are the top three things you look for in a boyfriend/girlfriend?",
    "What is your worst habit?",
    "How many stuffed animals do you own?",
    "What is your biggest insecurity?"
];

// Array of possible dare replies
const d = [
    "Do a free-style rap for the next minute.",
    "Let another person post a status on your behalf.",
    "Hand over your phone to another player who can send a single text saying anything they want to anyone they want.",
    "Let the other players go through your phone for one minute.",
    "Smell another player's armpit",
    "Smell another player's barefoot.",
    "Tell everyone your honest opinion of the person who sent this command."
];

// Handle all commands here
client.on('message', message => {

    // Don't reply to itself
    if (message.author.id === client.user.id) return;

    // If there is no + (prefix) at the beginning of the message, exit function
    if (!message.content.startsWith(prefix)) return;

    // Remove the prefix from the message -> our command
    const command = message.content.substring(prefix.length);

    // Match the command
    if (command === "t") { // Truth
        const truth = t[Math.floor(Math.random() * t.length)];
        message.channel.send(truth);
    } else if (command === "d") { // Dare
        const dare = d[Math.floor(Math.random() * d.length)];
        message.channel.send(dare);
    } else if (command === "help") { // Help

        const help = new Discord.MessageEmbed()
            .setColor('#72dfa3')
            .setTitle(`Truth Or Dare`)
            .addFields(
                { name: '``+help``', value: 'For help' },
                { name: '``+t``', value: 'For Truth' },
                { name: '``+d``', value: 'For Your Dare' },
                { name: '``Created By``', value: 'AlpHa Coder [Labib Khan]' },
            )
            .setTimestamp()
            .setFooter(`${message.author.username}`, message.author.displayAvatarURL());

        message.channel.send(help);

    } else { // No match found, invalid command
        message.channel.send("Invalid command, type `+help` for help.");
    }

});