message: check the given hash exists in the storage
diff: @@ -31,7 +31,9 @@ impl<'a> ReverseIter<'a> {
  None => return Err(Error::InvalidHeaderHash(bh.iter().cloned().collect())),
  Some(hh) => hh
  };
  - // TODO: check location of the hash actually exists
  + if let None = block_location(storage, hh.bytes()) {
  + return Err(Error::HashNotFound(hh.into_bytes()));
  + }
  let ri = ReverseIter {
  storage: storage,
  current_block: Some(hh)
  