message: fix default block_exists
diff: @@ -114,6 +114,9 @@ pub trait BlockStore: std::marker::Sized {
  /// Check whether a block exists.
  fn block_exists(&self, block_hash: &<Self::Block as Block>::Id) -> Result<bool, Error> {
  + if block_hash == &self.get_genesis_hash() {
  + return Ok(true);
  + }
  match self.get_block_info(block_hash) {
  Ok(_) => Ok(true),
  Err(Error::BlockNotFound) => Ok(false),
  