Set errors on model

This makes it possible to handle server side errors on the client side.

Function was copied from the ActiveModelAdapter (https://github.com/emberjs/data/blob/v1.0.0-beta.8/packages/activemodel-adapter/lib/system/active_model_adapter.js#L102) and modified to work with DRF.
Fixed test
Merge pull request #88 from chripede/patch-1

Set errors on model