Proper fix for openLbr v6ToInternet dispatch handling

openLbr.py -- Do not check return value of dispatch to v6ToInternet event.
              An absence of dispatch really is a configuration error that
              should be caught at setup time.
openTunLinux.py -- Update v6ToInternet event handling to raise an exception
                   if there is an error writing to the TUN interface. This
                   technique is how openTunWindows.py works already.