fix: error message passed to custom page

[adipasquale]

Reasoning 💡

I encountered a bug when using a custom page: the error querystring param is always undefined. This is a potential fix.

Steps to reproduce the bug on main:

1. set this config in [...nextauth].ts file like:

import { PrismaAdapter } from "@next-auth/prisma-adapter";
import NextAuth from "next-auth";
import EmailAuthProvider from "next-auth/providers/email";
import { PrismaClient } from "@prisma/client";

export default NextAuth({
  adapter: PrismaAdapter(new PrismaClient()),
  callbacks: {
    signIn({ user }) {
      if (typeof user.email !== "string") return false;
      return (user.email.endsWith("lycos.fr"))
    },
  },
  pages: {
    error: "/auth/error",
  },
  providers: [
    EmailAuthProvider({
      from: process.env.EMAIL_FROM,
      server: process.env.EMAIL_SERVER
    }),
  ],
});

2. set EMAIL_FROM and EMAIL_SERVER to variables that you can use (maybe use maildev)
3. try to login using the email provider and an email address different from *@lycos.fr

Checklist 🧢

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• Documentation
• Tests
• Ready to be merged

Affected issues 🎟

[adipasquale]

woups sorry for the dup this was fixed by #3141