Django doesn't respond with a form validation error
when a UniqueConstraint is violated
during the save process of an InlineAdmin formset.

How to reproduce

A pytest test function uses Django's test client
to save a Pizza object with the same Topping object twice,
leading to an IntegrityError:

python -m venv .venv
source .venv/bin/activate.sh
pip install -r requirements.txt
pytest 

Alternatively, use the admin and reproduce the steps manually:

python -m venv .venv
source .venv/bin/activate.sh
pip install -r requirements.txt
./manage.py migrate
./manage.py createsuperuser 
./manage.py runserver

Use the admin to create a new Topping object.
Then go to the Pizza form and add the previously created topping twice
in the inline admin formset.
Hit 'save' and you should see an IntegrityError.