Differences error handling behaviour between mongoose versions 4 & 5

index.js

//This is intended to illustrate differences error handling behaviour
//between mongoose versions 4 & 5

var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mongoosetest', { useMongoClient: true });

var AnimalSchema = new mongoose.Schema({
    name: String
});

//This may seem silly, but is contrived to illustrate the problem
AnimalSchema.statics.foo = function (callback) {
    var self = this;

    var animalId = new mongoose.mongo.ObjectId('5a990a276b84001e1061c5ba'); //Exists in collection

    this.findById(animalId, {name : 1}, function(err, animal) {
        
        if(err) {
            return callback(new Error('findById returned error: ' + err.toString()));
        }

        throw new Error('This is a thrown exception');

        callback(err);
    });
};

var Animal = mongoose.model('Animal', AnimalSchema);

Animal.foo(function(err) {
    if(err) {
        console.log(err)
    } else {
        console.log("OK!");
    }
});