better error message

row.h

@@ -171,7 +171,7 @@ public:
 			case 5:
 				return (ColValue &) *std::get_if<ColBooleanValue>(&m_columns[i]);
 			default:
-				throw Exception("should not happen");
+				throw Exception("ColValue &operator[](int i), type index invalid :" + std::to_string(type_index));
 		}
 	}