aardbei_sync: Actually use passed endpoint, log connection errors

piket_server/aardbei_sync.py

     AardbeiPerson objects. Return a AardbeiSyncError if something fails."""
     try:
         resp: requests.Response = requests.get(
-            f"{AARDBEI_ENDPOINT}/api/groups/0/",
+            f"{endpoint}/api/groups/0/",
             headers={"Authorization": f"Group {token}"},
         )
         resp.raise_for_status()
 
-    except requests.ConnectionError:
+    except requests.ConnectionError as e:
+        log.exception("Can't connect to endpoint %s", endpoint)
         return AardbeiSyncError.CantConnect
 
     except requests.HTTPError: