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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<?xml version="1.0" encoding="ISO-8859-1" ?>

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<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"

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            xmlns:xhtml="http://www.w3.org/1999/xhtml">

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  <xsd:element name="Python">

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    <xsd:complexType>

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      <xsd:annotation>

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        <xsd:documentation>Formatted text according to parts of XHTML 1.1</xsd:documentation>

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      </xsd:annotation>

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      <xsd:sequence>

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        <xsd:any namespace="http://www.w3.org/1999/xhtml" processContents="lax"/>

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      </xsd:sequence>

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    </xsd:complexType>

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  </xsd:element>

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</xsd:schema>