Restore Security::ErrorDetail::detailEntry initialization (#2364)

Restore Security::ErrorDetail::detailEntry initialization (#2364)

2023 commit 4e143970 accidentally removed code that was setting
`detailEntry` data member, breaking `%ssl_error_descr` expansion:
`Security::ErrorDetail::printErrorDescription()` would always print
`[Not available]`.

Squid still printed non-configurable request-independent error code
_name_ correctly because the corresponding `printErrorCode()` method
only uses `detailEntry` as a performance optimization.

The effects of this fix are visible, for example, in generated
ERR_SECURE_CONNECT_FAIL error responses:

```diff
- <p>[Not available]: /CN=...</p>
+ <p>Certificate does not match domainname: /CN=...</p>
```

This is a Measurement Factory project.