misc: block: streamline #[source] and Error impl

This streamlines the Error implementation in the Cloud Hypervisor code
base to match the remaining parts so that everything follows the agreed
conventions. These are leftovers missed in the previous commits.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

block/src/fcntl.rs

@@ -13,35 +13,25 @@
 //!
 //! [0]: <https://apenwarr.ca/log/20101213>.
 
-use std::error::Error;
-use std::fmt::{Debug, Display, Formatter};
+use std::fmt::Debug;
 use std::io;
 use std::os::fd::{AsRawFd, RawFd};
 
+use thiserror::Error;
+
 /// Errors that can happen when working with file locks.
-#[derive(Debug)]
+#[derive(Error, Debug)]
 pub enum LockError {
     /// The file is already locked.
     ///
     /// A call to [`get_lock_state`] can help to identify the reason.
+    #[error("The file is already locked")]
     AlreadyLocked,
     /// IO error.
-    Io(io::Error),
+    #[error("The lock state could not be checked or set: {0}")]
+    Io(#[source] io::Error),
 }
 
-impl Display for LockError {
-    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
-        match self {
-            LockError::AlreadyLocked => f.write_str("The file is already locked"),
-            LockError::Io(e) => {
-                write!(f, "{e}")
-            }
-        }
-    }
-}
-
-impl Error for LockError {}
-
 /// Commands for use with [`fcntl`].
 #[allow(non_camel_case_types)]
 enum FcntlArg<'a> {