Fix swapped errors for frozen/non-frozen dataclass inheritance

[nawrazi]

There is a mix-up in error messages related to frozen and non-frozen dataclass inheritance.

Currently:

• Inheriting a frozen dataclass from a non-frozen one incorrectly gives the error:
  Cannot inherit non-frozen dataclass from a frozen one

• Inheriting a non-frozen dataclass from a frozen one gives the error:
  Cannot inherit frozen dataclass from a non-frozen one

This PR swaps these two error messages and rephrases them for better clarity.

[brianschubert]

Thanks for the PR!

I think the current phrasing is a bit confusing. I'd suggest rewording these as Frozen dataclass cannot inherit from a non-frozen dataclass and vice versa.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Nice fix, thank you!