Invisible error for first paediatric assessment date before cohort 5

[mbarton]

I've seen someone in the production logs try to set a first paediatric assessment date in 2016. We don't want to accept that on the platform as it's not in a cohort open for submission. But for the user it silently fails - they click the button and nothing happens.

It also causes an unhandled exception in the backend:

django-1   |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
django-1   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django-1   |   File "/app/epilepsy12/decorator.py", line 398, in wrapper
django-1   |     return view(request, *args, **kwargs)
django-1   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django-1   |   File "/app/epilepsy12/decorator.py", line 309, in wrapper
django-1   |     return view(request, *args, **kwargs)
django-1   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django-1   |   File "/usr/local/lib/python3.11/site-packages/django/contrib/auth/decorators.py", line 60, in _view_wrapper
django-1   |     return view_func(request, *args, **kwargs)
django-1   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django-1   |   File "/app/epilepsy12/views/registration_views.py", line 619, in first_paediatric_assessment_date
django-1   |     validate_and_update_model(
django-1   |   File "/app/epilepsy12/common_view_functions/validate_form_update_model.py", line 149, in validate_and_update_model
django-1   |     child_cohort_data = cohorts_and_dates(
django-1   |                         ^^^^^^^^^^^^^^^^^^
django-1   |   File "/app/epilepsy12/general_functions/cohort_number.py", line 154, in cohorts_and_dates
django-1   |     "grace_cohort": dates_for_cohort(cohort=submitting_cohort_number - 1),
django-1   |                                             ~~~~~~~~~~~~~~~~~~~~~~~~~^~~
django-1   | TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'
django-1   | ERROR [django.server] "POST /case/1/first_paediatric_assessment_date HTTP/1.1" 500 104032

We should fix the crash and ensure we show a useful error message to the user