message: iostats: define its own error code
diff: @@ -11,6 +11,8 @@ use std::sync::atomic::{AtomicBool, AtomicIsize, AtomicUsize, Ordering};
  use std::sync::{Arc, RwLock};
  use std::time::SystemTime;
  +use serde_json::Error as SerdeError;
  +
  pub type Inode = u64;
  #[derive(PartialEq, Copy)]
  @@ -29,6 +31,12 @@ pub enum StatsFop {
  Max,
  }
  +#[derive(Debug)]
  +pub enum IoStatsError {
  + NoCounter,
  + Serialize(SerdeError),
  +}
  +
  impl Clone for StatsFop {
  fn clone(&self) -> Self {
  *self
  