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'

<?xml version='1.0' encoding='utf-8'?>
<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <variables/>
  <globals>
    <xhtml:p><![CDATA[import threading, time

MyT = None
Stop = False

def StartLog():
    global MyT
    MyT=threading.Thread(target = DoLog)
    MyT.start()

def DoLog():
    global Stop
    while not Stop:
        PLCObject.LogMessage("Python side Logging (PLC is %s)"%PLCObject.PLCStatus)
        time.sleep(0.3)

def StopLog():
    global MyT,Stop
    Stop=True

_runtime_init.append(StartLog)
_runtime_cleanup.append(StopLog)
]]></xhtml:p>
  </globals>
  <init>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </init>
  <cleanup>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </cleanup>
  <start>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </start>
  <stop>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </stop>
</PyFile>