message: storage: print explicit error message
diff: @@ -975,9 +975,12 @@ impl BlobDevice {
  for io_vec in io_vecs.iter() {
  if let Some(blob) = self.get_blob_by_iovec(io_vec) {
  + // Prefetch errors are ignored.
  let _ = blob
  .prefetch(blob.clone(), &[], &io_vec.bi_vec)
  - .map_err(|_e| eio!("failed to prefetch blob data"));
  + .map_err(|e| {
  + error!("failed to prefetch blob data, {}", e);
  + });
  }
  }
  