- Fixed a potential hang bug in wait_for_receiver() that could occur
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.