message: Return error values when iterator feature is not available
diff: @@ -77,7 +77,7 @@ where
  {
  // get rid of unused argument warning
  let (_, _, _, _) = (ctx, start_ptr, end_ptr, order);
  - 0
  + -1
  }
  #[cfg(feature = "iterator")]
  do_scan::<S, Q>(ctx, start_ptr, end_ptr, order)
  @@ -90,7 +90,7 @@ where
  {
  // get rid of unused argument warning
  let (_, _, _) = (ctx, key_ptr, value_ptr);
  - 0
  + -1
  }
  #[cfg(feature = "iterator")]
  do_next::<S, Q>(ctx, key_ptr, value_ptr)
  