Polyfill: Improve error message

This error message wasn't taking string time zone slot values into
account, which are the most common case. We can make the error message
more specific for this case.

polyfill/lib/ecmascript.mjs

@@ -1574,7 +1574,9 @@ export function InterpretISODateTimeOffset(
     const offsetStr = FormatUTCOffsetNanoseconds(offsetNs);
     const timeZoneString = IsTemporalTimeZone(timeZoneRec.receiver)
       ? GetSlot(timeZoneRec.receiver, TIMEZONE_ID)
-      : 'time zone';
+      : typeof timeZoneRec.receiver === 'string'
+        ? timeZoneRec.receiver
+        : 'time zone';
     throw new RangeError(`Offset ${offsetStr} is invalid for ${dt} in ${timeZoneString}`);
   }
   // fall through: offsetOpt === 'prefer', but the offset doesn't match