message: hopefully improve error message a bit
diff: @@ -383,7 +383,7 @@ impl<'tcx> Stack {
  // We use that to determine where to put the new item.
  let granting_idx = self.find_granting(access, derived_from)
  .ok_or_else(|| InterpError::MachineError(format!(
  - "no item to reborrow for {:?} from tag {} found in borrow stack", new.perm, derived_from,
  + "trying to reborrow for {:?}, but parent tag {} does not have an appropriate item in the borrow stack", new.perm, derived_from,
  )))?;
  // Compute where to put the new item.
  