misc: vmm/api: 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

vmm/src/api/http/mod.rs

@@ -3,9 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-use core::fmt;
 use std::collections::BTreeMap;
-use std::fmt::Display;
 use std::fs::File;
 use std::os::unix::io::{IntoRawFd, RawFd};
 use std::os::unix::net::UnixListener;
@@ -21,6 +19,7 @@ use micro_http::{
 use once_cell::sync::Lazy;
 use seccompiler::{apply_filter, SeccompAction};
 use serde_json::Error as SerdeError;
+use thiserror::Error;
 use vmm_sys_util::eventfd::EventFd;
 
 use self::http_endpoint::{VmActionHandler, VmCreate, VmInfo, VmmPing, VmmShutdown};
@@ -41,39 +40,31 @@ pub mod http_endpoint;
 pub type HttpApiHandle = (thread::JoinHandle<Result<()>>, EventFd);
 
 /// Errors associated with VMM management
-#[derive(Debug)]
+#[derive(Error, Debug)]
 pub enum HttpError {
     /// API request receive error
-    SerdeJsonDeserialize(SerdeError),
+    #[error("Failed to deserialize JSON: {0}")]
+    SerdeJsonDeserialize(#[source] SerdeError),
 
     /// Attempt to access unsupported HTTP method
+    #[error("Bad Request")]
     BadRequest,
 
     /// Undefined endpoints
+    #[error("Not Found")]
     NotFound,
 
     /// Too many requests
+    #[error("Too Many Requests")]
     TooManyRequests,
 
     /// Internal Server Error
+    #[error("Internal Server Error")]
     InternalServerError,
 
     /// Error from internal API
-    ApiError(ApiError),
-}
-
-impl Display for HttpError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use self::HttpError::*;
-        match self {
-            BadRequest => write!(f, "Bad Request"),
-            NotFound => write!(f, "Not Found"),
-            TooManyRequests => write!(f, "Too Many Requests"),
-            InternalServerError => write!(f, "Internal Server Error"),
-            SerdeJsonDeserialize(serde_error) => write!(f, "{serde_error}"),
-            ApiError(api_error) => write!(f, "{api_error}"),
-        }
-    }
+    #[error("Error from API: {0}")]
+    ApiError(#[source] ApiError),
 }
 
 impl From<serde_json::Error> for HttpError {