TST: add messages to bare pytest raises in pandas/tests/io/pytables/test_timezones.py
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR adds messages to the bare pytest raises in pandas/tests/io/pytables/test_timezones.py. This PR references https://github.com/pandas-dev/pandas/issues/30999.
> Nice, thanks! looks good to me pending green

Thanks!

> feel free to take on more than one file in the same PR if the changes aren't too big

Will do, I think that will work for a couple of the other files. @MarcoGorelli thanks for your suggestion, I made the change and now all the checks pass. thanks @marktgraham 