[ruby] improve possible error message in postfork parent (grpc#33791)

The case of `!grpc_ruby_initial_pid()` can be a *cause* of the second
case, `!grpc_ruby_initial_thread`, so check the pid first.

src/ruby/ext/grpc/rb_grpc.c

@@ -429,16 +429,16 @@ static VALUE grpc_rb_postfork_parent(VALUE self) {
              "GRPC::postfork_parent can only be called once following a "
              "GRPC::prefork");
   }
-  if (!grpc_ruby_initial_thread()) {
-    rb_raise(rb_eRuntimeError,
-             "GRPC.postfork_parent needs to be called from the same thread "
-             "that GRPC.prefork (and fork) was called from");
-  }
   if (!grpc_ruby_initial_pid()) {
     rb_raise(rb_eRuntimeError,
              "GRPC.postfork_parent must be called only from the parent process "
              "after a fork");
   }
+  if (!grpc_ruby_initial_thread()) {
+    rb_raise(rb_eRuntimeError,
+             "GRPC.postfork_parent needs to be called from the same thread "
+             "that GRPC.prefork (and fork) was called from");
+  }
   grpc_ruby_init_threads();
   g_grpc_rb_prefork_pending = false;
   return Qnil;