Customised error pages for "error pages" = 500 does not work

Nobody has claimed this yet.

I tried setting up my own error page for the HTTP 500 error thrown by PHP. And it didn't work.

Steps on how to check it:
1) Install OLS on Ubuntu 20.04 according to the manual.
2) `cp /usr/local/lsws/Example/html/phpinfo.php /usr/local/lsws/Example/html/p.php`
3) Edit /usr/local/lsws/Example/html/p.php
```
\\phpinfo();
phpinfo():
```
4) `cp /usr/local/lsws/Example/html/error404.html /usr/local/lsws/Example/html/error500.html`
5) WebAdmin > Virtual Host Example > General > Customized Error Pages add "500 Internal Server Error" -> /error500.html
6) Restart Server
7) run `curl -v http://localhost:8088/p.php`
```
* Trying ::1:8088...
* TCP_NODELAY set
* connect to ::1 port 8088 failed: Połączenie odrzucone
* Trying 127.0.0.1:8088...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8088 (#0)
> GET /p.php HTTP/1.1
> Host: localhost:8088
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
< content-type: text/html; charset=UTF-8
< content-length: 0
< date: Sat, 13 Nov 2021 06:08:54 GMT
< server: LiteSpeed
< connection: Keep-Alive
<
* Connection #0 to host localhost left intact

```

As you can see, it does not load the prepared page with the error.

Reproduce the failure with /usr/local/lsws/Example/html/p.php, /error500.html, and the WebAdmin Customized Error Pages setting, then run the shown curl request against port 8088. Trace how the HTTP 500 response is generated and verify that the configured error500.html content is returned instead of an empty response.

Written by the indexing model from the issue text.