DocumentForm deletion within DocumentFormSet

[m-vdb]

Not sure that the method used to check if a form has been deleted is right in this file. Django formset use the following test:

if form in formset.deleted_forms

In the case when data doesn't validate and is part of a deleted form, form.cleaned_data does not exist, and the way we test it can lead to problems (and in the worst case won't do everything we want...).