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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#EDE (Engineering-Data-Exchange) - generated by Beremiz BACnet plugin;;;;;;;;;;;;;;;

     2

PROJECT_NAME;%(Project Name)s;;;;;;;;;;;;;;

     3

VERSION_OF_REFERENCEFILE;%(EDE file version)d;;;;;;;;;;;;;;

     4

TIMESTAMP_OF_LAST_CHANGE;%(Current Time)s;;;;;;;;;;;;;;

     5

AUTHOR_OF_LAST_CHANGE;%(Project Author)s;;;;;;;;;;;;;;

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VERSION_OF_LAYOUT;2.1;;;;;;;;;;;;;;

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#mandatory;mandatory;mandatory;mandatory;mandatory;optional;optional;optional;optional;optional;optional;optional;optional;optional;optional;optional

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#keyname;device obj.-instance;object-name;object-type;object-instance;description;present-value-default;min-present-value;max-present-value;settable;supports COV;hi-limit;low-limit;state-text-reference;unit-code;vendor-specific-address