Solution for JSONAPIAdapter.updateRecord, DS.InvalidError and record.errors: https://github.com/emberjs/data/issues/6173

adapters.application.js

import DS from 'ember-data';
import RSVP from 'rsvp';

export default DS.JSONAPIAdapter.extend({
  updateRecord(store, type, snapshot) {
    if (snapshot.adapterOptions && snapshot.adapterOptions.withError) {
      return RSVP.reject(new DS.InvalidError([
        {
          detail: 'This name is reserved',
          source: {pointer: '/data/attributes/name'},
        },
      ]));
    } else {
      return this._super(store, type, snapshot); 
    }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  actions: {
    saveWithError() {
      this.model.save({adapterOptions: {withError: true}}); 
    },
  },
});

models.my-model.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  name: attr('string'),
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.store.push({
      data: {
        id: '1',
        type: 'my-model',
        attributes: {
          name: 'Foo', 
        },
      },
    });
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>

<p>
  model.isValid: {{model.isValid}}
</p>

<p>
  model.name: {{model.name}}
</p>

<p>
  model.errors.name.firstObject.message: {{model.errors.name.firstObject.message}}
</p>

<p>
  model.errors.length: {{model.errors.length}}
</p>

<p>
  <button {{action 'saveWithError'}}>
    Save with error
  </button>
</p>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.11.0-beta.1"
  }
}