fix(model): avoid TypeError if insertMany() fails with error that does not have writeErrors property

[vkarpov15]

Fix #13531

Summary

This issue is a bit tricky to test, but the idea is that, in the insertMany() error handler, the error may not necessarily have a writeErrors property. I was able to repro with the following script by shutting down my MongoDB server in the 10 seconds between "Sleep" and "Executing".

'use strict';

const mongoose = require('mongoose');

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://127.0.0.1:27017/mongoose_test', { serverSelectionTimeoutMS: 5000 });
  const Test = mongoose.model('Test', mongoose.Schema({ name: String }));
  await Test.init();

  console.log('Sleep');
  await new Promise(resolve => setTimeout(resolve, 10000));

  console.log('Executing');
  await Test.insertMany([{ name: 'foo' }]);
  console.log('Done');
}

With this change, we'll assume that all documents failed to insert if there's an error with no writeErrors property.

Examples