Clean validation problem

Hi,
I have noticed that clean method that should be override in order to allow custom validation on save() isn't called before the basic validations.

Suppose we have a required field that we want to perform a custom validation on it, an expected behavior would be that the clean method will be called after the validation of the required field. The reality is that the clean method called before the basic validation - so in order to add a custom validation we have to check if the required field isn't None and so as the rest of the basic validation.

PR for that?

Start by tracing save() validation and the clean method described in the issue, focusing on the order of custom validation versus required-field checks. Reproduce the case with a required field and custom validation, then verify that clean runs after the basic validations and that the resulting validation behavior matches the expected ordering.