message: Add block_exists utility function
diff: @@ -261,6 +261,14 @@ pub fn block_read(storage: &Storage, hash: &HeaderHash) -> Result<RawBlock> {
  block_read_location(storage, &block_location(storage, hash)?, hash)
  }
  +pub fn block_exists(storage: &Storage, hash: &HeaderHash) -> Result<bool> {
  + match block_location(storage, hash) {
  + Ok(_) => Ok(true),
  + Err(Error::HashNotFound(_)) => Ok(false),
  + Err(err) => Err(err),
  + }
  +}
  +
  enum ReverseSearch {
  Continue,
  Found,
  