Added notifications about error-exit values:
- The receiver notifies the generator if it is exiting with an error,
and then, if it is a server, waits around for the generator to die.
This ensures that the client side has time to read the error.
- The generator or sender will notifiy the other side of the transfer of
an error-exit value if protocol 31 is in effect. This will get rid of
some "connection unexpectedly closed" errors that are really expected
events due to a fatal exit on the other side.