message: chain-storage: Implement trait Error for the Error type
diff: +use std::{error, fmt};
  +
  #[derive(Debug, Clone, Eq, PartialEq)]
  pub enum Error {
  BlockNotFound, // FIXME: add BlockId
  }
  +
  +impl fmt::Display for Error {
  + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  + match self {
  + Error::BlockNotFound => write!(f, "block not found"),
  + }
  + }
  +}
  +
  +impl error::Error for Error {}
  