if the io_flush() call happened to read the last message from the
  receiver, causing the read_msg_fd() call to deadlock.
- Fixed an error-looping problem when the server-side receiver failed
  to send a message down the error-msg pipe:  we no longer try to send
  a new error about this new failure down the same failing pipe.
- Make sure that we stop any deferring of forwarded messages in the
  generator when we are exiting with an error.