standardize json error messages via the json_api renderer.

Linked to #61
I don't think passing the error would necessarily add kruft. For example the `not_found` method out be change to look like: 
``` ruby
  def update
    response_status, response_body = begin
      user = User.find(params[:id])
      authorize user
      user.update!(request_update_attributes(user))
      [ :ok, UserSerializer.resource(user) ]
    rescue PatchResourceError, ActiveRecord::RecordInvalid => e
      [ :bad_request, e]
    end
    render status: response_status, json_api: response_body
  end
```

And the render method:
``` ruby
def self.format_response_body(obj, options)
  response = obj.is_a?(Exception) ? { errors: [ message: obj.message ] } : obj
  response.to_json
end
```