DRF serializers.Serializer.errors must mimic Django forms.Form.errors

[edu...@coins.ph]

Starting from version 1.7, Django supports new API for validators and errors.

https://docs.djangoproject.com/en/1.8/ref/forms/validation/#raising-validationerror

https://docs.djangoproject.com/en/1.8/ref/forms/api/#django.forms.Form.errors.as_json

Client side developers could have a lot of benefits from error responses with error codes.

[Tom Christie]

See also https://github.com/tomchristie/django-rest-framework/pull/3775