Better error message

src/simstadt/results/energy_grid.py

@@ -20,7 +20,7 @@ class EnergyGridResults(SimStadtResults):
 
     def _parse_results(self) -> pd.DataFrame:
         jsons = self.get_all_by_extension(".json")
-        assert len(jsons) == 1
+        assert len(jsons) == 1, f"Exactly one JSON file should have been written: {len(jsons)} were found."
         json_output = jsons[0]
         with open(json_output, encoding="utf-8") as out:
             content = json.load(out)