Config should be validated with clearer errors

從 mautrix/util/config.py 開始，特別查看 _recursive_get，以及 mautrix_telegram/config.py，其中 bridge.filter.mode 會在設定期間被複製。重現報告中所示的啟動失敗，並讓無效的巢狀設定產生能指出問題的錯誤，而不是 NoneType traceback。

This doesn't tell me what is wrong:

Feb  3 10:15:20 integrations systemd[1]: Started Telegram Bridge.
Feb  3 10:15:22 integrations python[16389]: Traceback (most recent call last):
Feb  3 10:15:22 integrations python[16389]:   File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
Feb  3 10:15:22 integrations python[16389]:     "__main__", mod_spec)
Feb  3 10:15:22 integrations python[16389]:   File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
Feb  3 10:15:22 integrations python[16389]:     exec(code, run_globals)
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix_telegram/__main__.py", line 113, in <module>
Feb  3 10:15:22 integrations python[16389]:     TelegramBridge().run()
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/bridge/bridge.py", line 92, in run
Feb  3 10:15:22 integrations python[16389]:     self._prepare()
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/bridge/bridge.py", line 99, in _prepare
Feb  3 10:15:22 integrations python[16389]:     self.prepare_config(args.config, args.registration, args.base_config)
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/bridge/bridge.py", line 122, in prepare_config
Feb  3 10:15:22 integrations python[16389]:     self.config.update()
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 152, in update
Feb  3 10:15:22 integrations python[16389]:     self.do_update(ConfigUpdateHelper(base, self))
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix_telegram/config.py", line 146, in do_update
Feb  3 10:15:22 integrations python[16389]:     copy("bridge.filter.mode")
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 112, in copy
Feb  3 10:15:22 integrations python[16389]:     if from_path in self.source:
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 57, in __contains__
Feb  3 10:15:22 integrations python[16389]:     return self.get(key, None) is not None
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 50, in get
Feb  3 10:15:22 integrations python[16389]:     return self._recursive_get(self._data, key, default_value)
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 45, in _recursive_get
Feb  3 10:15:22 integrations python[16389]:     return self._recursive_get(next_data, next_key, default_value)
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 45, in _recursive_get
Feb  3 10:15:22 integrations python[16389]:     return self._recursive_get(next_data, next_key, default_value)
Feb  3 10:15:22 integrations python[16389]:   File "/home/telegram/tgenv/lib/python3.6/site-packages/mautrix/util/config.py", line 46, in _recursive_get
Feb  3 10:15:22 integrations python[16389]:     return data.get(key, default_value)
Feb  3 10:15:22 integrations python[16389]: AttributeError: 'NoneType' object has no attribute 'get'
Feb  3 10:15:22 integrations systemd[1]: telegram.service: Main process exited, code=exited, status=1/FAILURE