Fix the error message when deleting a media file in use

[seluianova]

Describe the Bug

When deleting a media file that is in use, a network error is displayed

Steps to Reproduce

1. Go to 'Media Library'
2. Try to delete a media file that is being used as the organisation icon
3. See error

Expected Behavior

I think we could display a more understandable (user-friendly) message.
Like: "This media file can't be deleted because it is in use"

Actual Behavior

The following error is displayed:
A network error has occurred. Please try again later.

Additional Information

Traceback

Dec 06 11:32:21 ERROR django.request - 500 Internal Server Error: /ajax/media/file/delete/
Traceback (most recent call last):
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
  response = get_response(request)
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
  response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/views/decorators/http.py", line 40, in inner
  return func(request, *args, **kwargs)
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
  return view_func(request, *args, **kwargs)
File "/home/tory/Integreat/integreat-cms/integreat_cms/api/decorators.py", line 115, in wrap
  return function(request, *args, **kwargs)
File "/home/tory/Integreat/integreat-cms/integreat_cms/cms/views/media/media_actions.py", line 361, in delete_file_ajax
  media_file.delete()
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/base.py", line 966, in delete
  collector.collect([self], keep_parents=keep_parents)
File "/home/tory/Integreat/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/deletion.py", line 302, in collect
  raise ProtectedError(
django.db.models.deletion.ProtectedError: ("Cannot delete some instances of model 'MediaFile' because they are referenced through protected foreign keys: 'Organization.icon'.", {<Organization (id: 1, slug: test-organization, region: augsburg)>})

[seluianova]

I am not sure about the expected result actually.
Maybe there should be no error in this case? And we should be able to delete used media files?

If so, what do we do with organisations, where the logo is mandatory, and the deleted file was used as a logo?

Or maybe we should disable the "delete" button in this case, and show a tooltip message?

[MizukiTemma]

If so, what do we do with organisations, where the logo is mandatory, and the deleted file was used as a logo?

That's why we want to prevent media files from being deleted when they are used. But I agree with you that just showing an error is not user friendly 😅

Or maybe we should disable the "delete" button in this case, and show a tooltip message?

I think disabling is the best. The reason can be explained like for archiving a mirrored page: