fix: Avoid return from finally: block

This fixes a SyntaxWarning on Python 3.14.

```
❯ uvx --no-cache --python 3.14.0 --with posthog==6.7.11 python -c "import posthog"
Installed 11 packages in 5ms
.../lib/python3.14/site-packages/posthog/consumer.py:92: SyntaxWarning: 'return' in a 'finally' block
  return success
````

Author: jodal

posthog/consumer.py

@@ -84,12 +84,16 @@ def upload(self):
             self.log.error("error uploading: %s", e)
             success = False
             if self.on_error:
-                self.on_error(e, batch)
+                try:
+                    self.on_error(e, batch)
+                except Exception as e:
+                    self.log.error("on_error handler failed: %s", e)
         finally:
             # mark items as acknowledged from queue
             for item in batch:
                 self.queue.task_done()
-            return success
+
+        return success
 
     def next(self):
         """Return the next batch of items to upload."""