Fix swapped errors for frozen/non-frozen dataclass inheritance (python#18918)

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

Author: nawrazi

mypy/plugins/dataclasses.py

@@ -359,12 +359,12 @@ def transform(self) -> bool:
 
         if decorator_arguments["frozen"]:
             if any(not parent["frozen"] for parent in parent_decorator_arguments):
-                self._api.fail("Cannot inherit frozen dataclass from a non-frozen one", info)
+                self._api.fail("Frozen dataclass cannot inherit from a non-frozen dataclass", info)
             self._propertize_callables(attributes, settable=False)
             self._freeze(attributes)
         else:
             if any(parent["frozen"] for parent in parent_decorator_arguments):
-                self._api.fail("Cannot inherit non-frozen dataclass from a frozen one", info)
+                self._api.fail("Non-frozen dataclass cannot inherit from a frozen dataclass", info)
             self._propertize_callables(attributes)
 
         if decorator_arguments["slots"]:

test-data/unit/check-dataclasses.test

@@ -260,15 +260,15 @@ class FrozenBase:
     pass
 
 @dataclass
-class BadNormalDerived(FrozenBase):  # E: Cannot inherit non-frozen dataclass from a frozen one
+class BadNormalDerived(FrozenBase):  # E: Non-frozen dataclass cannot inherit from a frozen dataclass
     pass
 
 @dataclass
 class NormalBase:
     pass
 
 @dataclass(frozen=True)
-class BadFrozenDerived(NormalBase):  # E: Cannot inherit frozen dataclass from a non-frozen one
+class BadFrozenDerived(NormalBase):  # E: Frozen dataclass cannot inherit from a non-frozen dataclass
     pass
 
 [builtins fixtures/dataclasses.pyi]