Improve authentication error handling in config flow

[pdcastro]

Before this PR, entering an incorrect username or password in the config flow would cause the form to display “Unexpected error”, while the hass logs would print “Unexpected exception” and a stack trace:

2024-02-09 18:25:24.786 ERROR (MainThread) [custom_components.volkswagen_we_connect_id.config_flow] Unexpected exception
Traceback (most recent call last):
  File "/home/vscode/.homeassistant/custom_components/volkswagen_we_connect_id/config_flow.py", line 62, in async_step_user
    info = await validate_input(self.hass, user_input)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vscode/.homeassistant/custom_components/volkswagen_we_connect_id/config_flow.py", line 37, in validate_input
    await hass.async_add_executor_job(we_connect.login)
  File "/usr/local/lib/python3.12/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vscode/.local/lib/python3.12/site-packages/weconnect/weconnect.py", line 187, in login
    self.__session.login()
  File "/home/vscode/.local/lib/python3.12/site-packages/weconnect/auth/we_connect_session.py", line 76, in login
    response = self.doWebAuth(authorizationUrl)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vscode/.local/lib/python3.12/site-packages/weconnect/auth/we_connect_session.py", line 198, in doWebAuth
    raise AuthentificationError(f'Error during login, account {self.sessionuser.username} does not exist')
weconnect.errors.AuthentificationError: Error during login, account [email protected] does not exist

The last line above might also be:

weconnect.errors.AuthentificationError: Error during login, email invalid

After this PR, nothing is printed in the hass logs (at least not with my default log level), and the user interface displays a seemingly hass-standard “Invalid authentication” message:

The reason why I was messing with that is that I was testing my code changes in another PR, #245.

[pdcastro]

That spelling is not a typo of mine! It is how it reads in the library:
https://github.com/tillsteinbach/WeConnect-python/blob/v0.59.5/weconnect/errors.py#L17

[pdcastro]

This exception class was defined but not raised anywhere, and therefore not capable of being caught through except InvalidAuth. I assume it was connected to that comment earlier in the file:

# TODO: ADD Validation on credentials