Make logging of rendezvous to private address quieter

In legacy/trac#8976 (moved), we blocked rendezvous from hidden services to private addresses.

But the warning is log_warn LD_REND, when arma wonders if it should be a protocol warning to avoid excessive logging on busy services:

  log_warn(LD_REND, "%s on circ %u", err_msg,
           (unsigned)circuit->base_.n_circ_id);

There's also a whitespace issue we might as well fix while we're there:

  if (err_msg_out) *err_msg_out = err_msg;
  else tor_free(err_msg);