Fix crash in runtime if PSK secret is missing

./Beremiz_service.py -s $PWD/psk2.txt -n beremiz /tmp/beremiz
Beremiz_service: 1.2-1378c18402c3+

Traceback (most recent call last):
File "./Beremiz_service.py", line 511, in <module>
ensurePSK(servicename, PSKpath)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/Stunnel.py", line 32, in ensurePSK
PSKgen(ID, PSKpath)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/Stunnel.py", line 23, in PSKgen
call(restart_stunnel_cmdline)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/spawn_subprocess.py", line 116, in call
pid = posix_spawn.posix_spawnp(cmd[0], cmd)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 120, in posix_spawnp
return _posix_spawn(lib.posix_spawnp, *args, **kwargs)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 111, in _posix_spawn
_check_error(res, path)
File "/home/developer/.local/lib/python2.7/site-packages/posix_spawn/_impl.py", line 10, in _check_error
raise OSError(errno, os.strerror(errno), path)
OSError: [Errno 2] No such file or directory: '/etc/init.d/S50stunnel'

// import Nevow.Athena

function init() {
  Nevow.Athena.Widget.fromAthenaID(1).callRemote('HMIexec', 'HMIinitialisation');
}

WebInterface.PLC = Nevow.Athena.Widget.subclass('WebInterface.PLC');
WebInterface.PLC.method(
	 'updateHMI',
	 function (self, data) {
	   d = self.callRemote('getPLCElement');
	   d.addCallback(
			 function liveElementReceived(le) {
				d2 = self.addChildWidgetFromWidgetInfo(le);
				d2.addCallback(
						function childAdded(widget) {
						var node = self.nodeById('content');
						node.replaceChild(widget.node, node.getElementsByTagName('div')[0]);
						init();
						});
				});
	   });

Divmod.Base.addLoadEvent(init);