Update model.py

Author: barneydobson

wsimod/orchestration/model.py

@@ -261,6 +261,8 @@ def load(self, address, config_name="config.yml", overrides={}):
         unified_data_file = data.get("unified_data_file")
         if unified_data_file and PARQUET_AVAILABLE:
             self._load_unified_data(address, unified_data_file, nodes, data)
+        elif unified_data_file and not PARQUET_AVAILABLE:
+            raise ValueError("Parquet support is required for unified data loading")
         else:
             self._load_individual_files(address, nodes, data)
         arcs = data.get("arcs", {})