[ENH] improved simple assertion error message in evalutation/functions.py (#1600)

#### Details
fixed a simple assertion error message in `evalutation/functions.py`

Author: EmanAbdelhaleem

openml/evaluations/functions.py

@@ -231,8 +231,9 @@ def __list_evaluations(api_call: str) -> list[OpenMLEvaluation]:
             f'Error in return XML, does not contain "oml:evaluations": {evals_dict!s}',
         )
 
-    assert isinstance(evals_dict["oml:evaluations"]["oml:evaluation"], list), type(
-        evals_dict["oml:evaluations"],
+    assert isinstance(evals_dict["oml:evaluations"]["oml:evaluation"], list), (
+        "Expected 'oml:evaluation' to be a list, but got"
+        f"{type(evals_dict['oml:evaluations']['oml:evaluation']).__name__}. "
     )
 
     uploader_ids = list(