ERR: Check that dtype_backend is valid
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

this sits on top of the actual pr
Quick thoughts:
- Generally, nullables are still slower than NumPy itself (and need more memory)

Implementation wise:
- There is still no global version to opt in, similar to pyarrow backend
- NA vs NaN in Float64/32 is still open
- Couple of other things

Those things have to be solved before we can start discussing it Yep, makes sense. On a second thought, if we think pyarrow will eventually be the default, not sure if it's worth to change to nullable first, and then change it again.

In any case, great work with the new interface to change the backend, thanks for taking care of it! Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 2.0.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -x -m1 dba0f66785aff795f30afbf9135771d3c11b5a67
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #51871: ERR: Check that dtype_backend is valid'
```

4. Push to a named branch:

```
git push YOURFORK 2.0.x:auto-backport-of-pr-51871-on-2.0.x
```

5. Create a PR against branch 2.0.x, I would have named this PR:

> "Backport PR #51871 on branch 2.0.x (ERR: Check that dtype_backend is valid)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 Yeah that's something else to consider. I just wanted to express that there is some work to do before we can start discussing these topics