Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
posthog/client.py:1353-1370
**Duplicated message content (OnceAndOnlyOnce)**

The error message body — `"Error loading feature flags: {e.message}. Please verify both your project_api_key and personal_api_key. More information: https://posthog.com/docs/api/overview"` — is constructed twice: once as a `%s` format string for the log call, and again as an f-string for the re-raised `APIError`. Extracting it into a local variable eliminates the duplication and keeps the two outputs in sync automatically.

```suggestion
                detail = (
                    f"Error loading feature flags: {e.message}. "
                    "Please verify both your project_api_key and personal_api_key. "
                    "More information: https://posthog.com/docs/api/overview"
                )
                self.log.error("[FEATURE FLAGS] %s", detail)
                self.feature_flags = []
                self.group_type_mapping = {}
                self.cohorts = {}

                if self.flag_cache:
                    self.flag_cache.clear()

                if self.debug:
                    raise APIError(status=401, message=detail)
```