Update cubids.py

Author: tsalo

cubids/cubids.py

@@ -1061,8 +1061,10 @@ def get_param_groups_from_entity_set(self, entity_set):
         matching_files = self.layout.get(
             return_type="file", scope="self", regex_search=True, **key_entities
         )
+        if not matching_files:
+            raise Exception(f"No files found for entity set: {entity_set}")
 
-        # ensure files who's entities contain key_entities but include other
+        # ensure files whose entities contain key_entities but include other
         # entities do not also get added to matching_files
         to_include = []
         for filepath in matching_files: