How do I return an "invalid login" message when user logs in?

So far I have this, which works and allows the user to successfully login with the given credentials:

const ID = ‘’;
const PASS = ‘pass’;

Meteor.startup(() => {
if (!Accounts.findUserByEmail(ID)) {
email: ID,
password: PASS,

But I want to return an error that displays a message saying that the email ID is incorrect, or and error saying that the password is incorrect, if the user doesn’t put the correct email id or password. How do I do that and also keep the code I have here?

(P.S. I have to keep ^that code as per the instructions)