Use variadic template args for propagate_error()

Let's see if bots happy.

Not sure where else we can use these.  Mm.  Maybe in hb_invoke().

src/hb-serialize.hh

@@ -126,11 +126,8 @@ struct hb_serialize_context_t
   template <typename T> bool propagate_error (T &&obj)
   { return propagate_error (!hb_deref_pointer (obj).in_error ()); }
 
-  template <typename T1, typename T2> bool propagate_error (T1 &&o1, T2 &&o2)
-  { return propagate_error (o1) && propagate_error (o2); }
-  template <typename T1, typename T2, typename T3>
-  bool propagate_error (T1 &&o1, T2 &&o2, T3 &&o3)
-  { return propagate_error (o1) && propagate_error (o2, o3); }
+  template <typename T1, typename... Ts> bool propagate_error (T1 &&o1, Ts &&...os)
+  { return propagate_error (o1) && propagate_error (os...); }
 
   /* To be called around main operation. */
   template <typename Type>