Fix a regression in pytest 8.0 where tracebacks get longer and longer when multiple tests fail due to a shared higher-scope fixture which raised.

Also fix a similar regression in pytest 5.4 for collectors which raise during setup.

The fix necessitated internal changes which may affect some plugins:
- ``FixtureDef.cached_result[2]`` is now a tuple ``(exc, tb)`` instead of ``exc``.
- ``SetupState.stack`` failures are now a tuple ``(exc, tb)`` instead of ``exc``.