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'

all:
	@echo "Please specify target 'source_list', 'template' or 'locales'"

template:
	@echo To generate translation template message.pot file:
	python mki18n.py -p --domain=Beremiz

locales:
	@echo "Generate .mo files for all languages:"
	python mki18n.py -m --moTarget=../locale --domain=Beremiz


source_list:
	echo @To "Generate list with source files: app.fil:"
	find .. -name "*.py" -exec grep -q '_(' {} ';' -print -o -name "*XSD*" -print -o -name "*.csv" -print | grep -v '/build/' | grep -v '/pyjs/' > app.fil
	echo "../plcopen/Additional_Function_Blocks.xml" >> app.fil
	echo "../plcopen/Standard_Function_Blocks.xml"   >> app.fil

transifex_init:
	tx init --skipsetup

transifex_pull:
	tx pull --all
	find . -name '*.po' -exec sed -i 's/charset=CHARSET/charset=UTF-8/' {} \;


.PHONY: all, template, locales, source_list