RESTAdapter : Convert errors object returned with a 422 in an Ember Object

Without this, after an error 422, the errors on the record are not an Ember Object. I believe the wanted behavior is to convert it to an Ember Object as [it](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/model/states.js#L396) [is](https://github.com/emberjs/data/blob/master/packages/ember-data/tests/integration/store_adapter_test.js#L291) use like one.

I would love any feedback on this if it is not relevant.