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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IEC 61131-3

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===========

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    IEC-61131 is a normative document provided by the standards organization IEC

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    (International Electrotechnical Commission) and describing a standard for

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    implementing programmable controllers.

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    The part 3 of this document (commonly named IEC 61131-3) specifies syntax and 

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    semantics for programming language for programmable controllers. Beremiz

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    implements all the languages described in this document. 

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    http://www.iec.eu

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PLCopen TC6

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===========

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    PLCopen is a vendor- and product-independent worldwide association defining

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    international standards for various topics related to control programming.

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    For this purpose, PLCopen has 6 technical committees.

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    The goal of the sixth committee (TC6) is to define a standard file format, 

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    based on XML, for exchanging programmables controllers programmed using 

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    IEC 61131-3 languages. Beremiz uses this file format for saving the PLC

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    programs of projects.

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    http://www.plcopen.org

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