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'

# SOME DESCRIPTIVE TITLE.
# Copyright (C) is 21st century inquisition
# This file is distributed under the same license as the Beremiz package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
msgid ""
msgstr ""
"Project-Id-Version: Beremiz 1.1\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2012-09-08 00:39\n"
"PO-Revision-Date: 2012-09-07 10:48+0100\n"
"Last-Translator: \n"
"Language-Team: \n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"X-Poedit-Language: French\n"
"X-Poedit-Country: FRANCE\n"
"X-Poedit-SourceCharset: utf-8\n"

# e69e0ee137df409290c6c6a5fbd512a6
#: ../../index.rst:4
msgid "Beremiz's documentation"
msgstr "Documentation de Beremiz"

# 7a5a9d62913b477b99421d5dc94a44be
#: ../../index.rst:6
msgid "Contents:"
msgstr "Contenu"

# 899ecb5f4ff84b8e98dbf5b1eaf7f393
#~ msgid "Indices and tables"
#~ msgstr "Index et tables"