catch in the UI a server restart (invalid secret)

[ReimarBauer]

When a server invalidates the secret then a running client can get a traceback on next action.
Also a traceback may happen on a server reboot, when the service is for a short moment offline.
Or on the client site on a instable network connection.

Fatal error in MSS 8.2.0 on Linux-6.2.0-10018-tuxedo-x86_64-with-glibc2.35
Python 3.10.8 | packaged by conda-forge | (main, Nov 22 2022, 08:23:14) [GCC 10.4.0]

Please report bugs in MSS to https://github.com/Open-MSS/MSS

Information about the fatal error:

Traceback (most recent call last):
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/site-packages/mslib/msui/mscolab.py", line 1668, in set_active_op_id
    self.load_wps_from_server()
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/site-packages/mslib/msui/mscolab.py", line 1793, in load_wps_from_server
    xml_content = self.request_wps_from_server()
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/site-packages/mslib/msui/mscolab.py", line 1782, in request_wps_from_server
    xml_content = json.loads(r.text)["content"]
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/home/reimar/mambaforge/envs/mssenv/lib/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)