Non-existent user shall result in exit

If the program runs into an error due to an non-existent user, it should abort and not continue. Currently, it's too easy to miss in the amount of logging messages.

[2026-09-01 15:32:25] ERROR: The user 'ue72907' configured as member of team 'NGE' does not exist on GitHub. Spelling error or did they rename themselves?
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/gh_org_mgr/_gh_org.py", line 131, in _resolve_gh_username
    gh_user: NamedUser = self.gh.get_user(username)  # type: ignore[assignment]  # ty:ignore[invalid-assignment]
                         ~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/MainClass.py", line 446, in get_user
    user = github.NamedUser.NamedUser(requester, url=url)
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/GithubObject.py", line 633, in __init__
    self.complete()
    ~~~~~~~~~~~~~^^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/GithubObject.py", line 678, in complete
    self._completeIfNeeded()
    ~~~~~~~~~~~~~~~~~~~~~~^^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/GithubObject.py", line 687, in _completeIfNeeded
    self._complete()
    ~~~~~~~~~~~~~~^^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/GithubObject.py", line 692, in _complete
    headers, data = self._requester.requestJsonAndCheck(
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        "GET", self._url.value, parameters=parameters, headers=self.__completeHeaders
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/Requester.py", line 658, in requestJsonAndCheck
    *self.__check(
     ~~~~~~~~~~~~^
        *self.requestJson(
        ^^^^^^^^^^^^^^^^^^
    ...<7 lines>...
        )
        ^
    ),
    ^
  File "/opt/hostedtoolcache/Python/3.14.7/x64/lib/python3.14/site-packages/github/Requester.py", line 897, in __check
    raise self.createException(status, responseHeaders, data)
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest", "status": "404"}

Start in gh_org_mgr/_gh_org.py at _resolve_gh_username, around the reported line 131, and trace how the missing-user error is returned to the organization-management flow. Reproduce the configuration with a nonexistent GitHub username and verify that processing aborts instead of continuing after the error.