refactor(utils): simplify HTTP error payload key lookup

Collapse the payload message lookup to the two conventional keys.

ariadne/utils/errors.py

@@ -12,7 +12,7 @@ def _runtime_error_detail(exc: Exception) -> str | None:
 
 def _http_message_from_payload(payload: object) -> str | None:
     if isinstance(payload, dict):
-        raw = payload.get("errorMessage") or payload.get("error") or payload.get("message")
+        raw = payload.get("error") or payload.get("message")
         if isinstance(raw, str) and raw.strip():
             return raw.strip()
     if isinstance(payload, str) and payload.strip():