Fix API exception response handling

For some reason when using `render text:` instead of `render json:` for this response, the status code being returned was 200 instead of the clearly explicitly defined 422 (:unprocessable_entity). I absolutely have no idea why!

app/controllers/api/base_controller.rb

@@ -73,7 +73,7 @@ module Api
     end
 
     def error_during_processing(exception)
-      render(text: { exception: exception.message }.to_json,
+      render(json: { exception: exception.message },
              status: :unprocessable_entity) && return
     end