Implement validateErrorHandler utility.

[rwjblue]

Ensures that Ember.onerror is setup that properly re-throws when testing.

This is intended to be used by test framework hosts (or other libraries) to ensure that Ember.onerror is properly configured. Without a check like this, Ember.onerror could easily swallow all errors and make it seem like everything is just fine (and have green tests) when in reality everything is on fire...

Example implementation for ember-qunit:

import { validateErrorHandler } from '@ember/test-helpers';

test('Ember.onerror is functioning properly', function(assert) {
  let result = validateErrorHandler();
  assert.ok(result.isValid, result.message);
});

Additional reading:

• more detailed writeup in [BUGFIX lts] Refactor / fix error handling scenarios. ember.js#15871
• ember-test-friendly-error-handler addon makes it very simple to implement a test friendly Ember.onerror