Conflict error for compound indexes is not correct

Steps to reproduce

1. Create a compound index for your collection:

const mongoose = require('mongoose')

const schema = new mongoose.Schema({
  // random fields used as an example
  firstId: { type: mongoose.Types.ObjectId },
  secondId: { type: mongoose.Types.ObjectId }
})

schema.index({ firstId: 1, secondId: 1 }, { unique: true })

1. Create two documents, both with the same firstId and secondId field values and inspect the error message returned from mongoose. It will look something like this: firstId: ObjectId(\'5b891cf30d74a9432c0ee036\'), : ObjectId(\'587775b068194a6b3c818dce\') already exists.

Expected behavior

Feathers mongoose should handle a 1100 or 11001 MongoDB error and display a useful error message for the user of the library

Actual behavior

This works fine for regular indexes, but when there is a compound index the error message is rather obscure.

System configuration

Tell us about the applicable parts of your setup.

Module versions (especially the part that’s not working): feathers-mongoose: 6.1.2 mongoose: 4.13.14

NodeJS version: 8.11.2

Operating System: Ubuntu 16.04

[daffl]

We probably could do the same thing the new Sequelize adapter is doing and add a hidden property with the original error (https://github.com/feathersjs-ecosystem/feathers-sequelize#migrating).

[daffl]

It basically just adds the original error in a Symbol error property (see https://github.com/feathersjs-ecosystem/feathers-sequelize/blob/master/lib/utils.js) so it can be accessed and used in the server-side error handler.