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.
Hello @marktgraham! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-12-11 16:27:05 UTC The reason the tests are failing is because I don't know what causes the difference between these two error messages:

```
if ("numpy" not in str(type(arr))) | (
     arr.dtype.name not in ["string"]
):
     msg = "index 3 is out of bounds for axis 0 with size 3"
else:
     msg = "index 3 is out of bounds for size 3"
```

The error message is `AssertionError: Regex pattern 'index 3 is out of bounds for axis 0 with size 3' does not match 'index 3 is out of bounds for size 3'.` i.e. it should be selecting the second one but it selects the first one.

I've tried a couple of different things, but neither has worked. I thought it was something to do with the number of dimensions but that doesn't seem to work either.

 Thanks @MarcoGorelli ! thanks @marktgraham 