misc: performance-metrics: 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

performance-metrics/src/performance_tests.rs

@@ -10,6 +10,7 @@ use std::time::Duration;
 use std::{fs, thread};
 
 use test_infra::{Error as InfraError, *};
+use thiserror::Error;
 
 use crate::{mean, PerformanceTestControl};
 
@@ -19,10 +20,13 @@ pub const FOCAL_IMAGE_NAME: &str = "focal-server-cloudimg-amd64-custom-20210609-
 pub const FOCAL_IMAGE_NAME: &str = "focal-server-cloudimg-arm64-custom-20210929-0-update-tool.raw";
 
 #[allow(dead_code)]
-#[derive(Debug)]
+#[derive(Error, Debug)]
 enum Error {
+    #[error("boot time could not be parsed")]
     BootTimeParse,
-    Infra(InfraError),
+    #[error("infrastructure failure: {0}")]
+    Infra(#[source] InfraError),
+    #[error("restore time could not be parsed")]
     RestoreTimeParse,
 }