title: Django admin return custom error message during model saving
body: <p>I would like to return some custom error messages in  <code>save_model</code> function of Django admin page.</p>
  
  <pre><code>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()
  </code></pre>
  
  <p>Of course, <code>Else</code> part isn't correct but I want to illustrate what I want.</p>
  
  <p>I am glad to suggest me a way or document to do that.
  Thanks</p>