message: Add missing error strings
diff: @@ -70,6 +70,11 @@ impl fmt::Display for Error {
  WrongTxProof => write!(f, "transaction proof is invalid"),
  WrongUpdateProof => write!(f, "update proof is invalid"),
  ZeroCoin => write!(f, "output with no credited value"),
  + WrongPreviousBlock => write!(f, "block has wrong parent"),
  + NonExistentSlot => write!(f, "slot does not have a leader"),
  + BlockDateInPast => write!(f, "block's slot or epoch is earlier than its parent"),
  + BlockDateInFuture => write!(f, "block is in a future epoch"),
  + WrongSlotLeader => write!(f, "block was not signed by the slot leader indicated in the genesis block"),
  }
  }
  }
  