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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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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from __future__ import absolute_import

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from __future__ import print_function

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import traceback

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import sys

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from runtime.Worker import worker

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MainWorker = worker()

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_PLCObjectSingleton = None

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def GetPLCObjectSingleton():

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    assert _PLCObjectSingleton is not None

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    return _PLCObjectSingleton

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def LogMessageAndException(msg, exp=None):

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    if exp is None:

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        exp = sys.exc_info()

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    if _PLCObjectSingleton is not None:

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        _PLCObjectSingleton.LogMessage(0, msg + '\n'.join(traceback.format_exception(*exp)))

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    print(msg)

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    traceback.print_exception(*exp)

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def CreatePLCObjectSingleton(*args, **kwargs):

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    global _PLCObjectSingleton

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    from runtime.PLCObject import PLCObject  # noqa # pylint: disable=wrong-import-position

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    _PLCObjectSingleton = PLCObject(*args, **kwargs)