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'

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// import Nevow.Athena

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function init() {

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  Nevow.Athena.Widget.fromAthenaID(1).callRemote('HMIexec', 'HMIinitialisation');

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}

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WebInterface.PLC = Nevow.Athena.Widget.subclass('WebInterface.PLC');

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WebInterface.PLC.method(

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	 'updateHMI',

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	 function (self, data) {

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	   d = self.callRemote('getPLCElement');

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	   d.addCallback(

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			 function liveElementReceived(le) {

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				d2 = self.addChildWidgetFromWidgetInfo(le);

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				d2.addCallback(

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						function childAdded(widget) {

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						var node = self.nodeById('content');

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						node.replaceChild(widget.node, node.getElementsByTagName('div')[0]);

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						init();

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						});

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				});

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	   });

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Divmod.Base.addLoadEvent(init);