message: fix when epoch does not exist anymore
diff: @@ -38,7 +38,16 @@ impl<'a> Iter<'a> {
  self.end = true;
  let next_epoch = self.from + 1;
  self.current = {
  - let epochref = epoch_read_pack(self.storage, next_epoch)?;
  + let epochref = match epoch_read_pack(self.storage, next_epoch) {
  + Err(err) => {
  + if err.kind() == ::std::io::ErrorKind::NotFound {
  + return Ok(None);
  + } else {
  + return Err(Error::IoError(err));
  + }
  + },
  + Ok(c) => c
  + };
  PackReader::init(self.storage, &epochref)
  };
  self.from = next_epoch;
  