title: How do i use validation in admin.py while overriding save_model() function?
body: <pre><code>Admin.py
  
  class CourseAdmin(admin.ModelAdmin):
  
  list_display = ('course_code', 'title', 'short' )
  
   def save_model(self, request, obj, form, change):
      import os
  
      #obj.author = request.user
      dir_name = obj.course_code
      path = settings.MEDIA_ROOT +os.sep+'xml'+os.sep+dir_name
  
      #if user updates course name then course would be renames
      if change:
          dir_name = Course.objects.get(pk=obj.pk).course_code
          src = settings.MEDIA_ROOT +os.sep+'xml'+os.sep+dir_name      
          os.rename(src,path)
      else:
          if not os.path.exists(path):
              os.makedirs(path)
              obj.save()
  
          else:
              raise ValidationError('Bla Bla')
  
  
  admin.site.register(Course, CourseAdmin)
  </code></pre>
  
  <p>when i raise  validation Error it doesn't work and shows error page with 
  Exception Type: Validation Error
  Exception Value:[u'Bla Bla']</p>