gh-90716: bugfixes and more tests for _pylong. (#99073)

* Properly decref on _pylong import error.
* Improve the error message on _pylong TypeError.
* Fix the assertion error in pydebug builds to be a TypeError.
* Tie the return value comments together.

These are minor followups to issues not caught among the reviewers on
https://github.com/python/cpython/pull/96673.