TST: Assert msg with pytest raises in pandas/tests/extension/base
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR adds messages to the bare pytest raises in reduce.py and getitem.py in pandas/test/extension/base. This PR references #30999.