Apply suggestions from code review

Co-authored-by: Daria <93913290+blondered@users.noreply.github.com>

Author: nsundalov

rectools/models/nn/transformers/base.py

@@ -651,7 +651,7 @@ def _model_from_checkpoint(
     def __getstate__(self) -> object:
         if self.is_fitted:
             if self.fit_trainer is None:
-                raise RuntimeError("expected to have fit_trainer set")
+                raise RuntimeError("Fitted model is expected to have `fit_trainer` set")
 
             with NamedTemporaryFile() as f:
                 self.fit_trainer.save_checkpoint(f.name)