Some error handling and more use cases support

Some changes needed to support new use cases and basic error handling.

This branch https://github.com/rockabox/rbx_campaigns/tree/jose/5380-add-creatives-as-nested-objects-to-adunits-endpoints needed:
* Use a source field with the same name than the final field. E.g. `display_unit` is an ID which is used as `source` to retrieve the final `display_unit` nested object.
* Support writing to the same fields