add more informative login error message

[ekvanox]

Right now the output is hard to understand:

Traceback (most recent call last):
  File "./wordfeudbot/main.py", line 1074, in <module>
    main()
  File "./wordfeudbot/main.py", line 743, in main
    wf.sessionid = wf.login(
  File "./wordfeudbot/main.py", line 62, in login
    if response.json()["status"] == "error":
  File "/usr/local/lib/python3.8/site-packages/requests/models.py", line 898, in json
    return complexjson.loads(self.text, **kwargs)
  File "/usr/local/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/local/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/lib/python3.8/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)