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'

from __future__ import absolute_import

import wx
from controls.IDBrowser import IDBrowser


class IDManager(wx.Dialog):
    def __init__(self, parent, ctr):
        self.ctr = ctr
        wx.Dialog.__init__(self,
                           name='IDManager', parent=parent,
                           title=_('URI Editor'),
                           style=wx.DEFAULT_DIALOG_STYLE | wx.RESIZE_BORDER,
                           size=(800, 600))
        # start IDBrowser in manager mode
        self.browser = IDBrowser(self, ctr)
        self.Bind(wx.EVT_CHAR_HOOK, self.OnEscapeKey)

    def OnEscapeKey(self, event):
        keycode = event.GetKeyCode()
        if keycode == wx.WXK_ESCAPE:
            self.EndModal(wx.ID_CANCEL)
        else:
            event.Skip()