Django admin return custom error message during model saving

I would like to return some custom error messages in save_model function of Django admin page.

class EmployerAdmin(admin.ModelAdmin):
  exclude = ('update_user','updatedate','activatedate','activate_user')

  def save_model(self, request, obj, form, change):
    if obj.department != None and obj.isDepartmentSuggested:
        obj.isDepartmentSuggested =False
    else:
       return "You don't set a valid department. Do you want to continue ?"

    obj.update_user = request.user
    obj.updatedate = datetime.datetime.now()
    obj.save()

Of course, Else part isn't correct but I want to illustrate what I want.

I am glad to suggest me a way or document to do that. Thanks