perf net: Add the endpoint name related to the perf net error (#18063)

In a perf test, one node will run speed test with all nodes. If there is
an error with a peer node, the peer node name is not included in the
error hence confusing the user.

This commit will add the peer endpoint string to the netperf error.

cmd/perf-tests.go

@@ -306,7 +306,7 @@ func netperf(ctx context.Context, duration time.Duration) madmin.NetperfNodeResu
 					defer wg.Done()
 					err := globalNotificationSys.peerClients[index].DevNull(ctx, r)
 					if err != nil {
-						errStr = err.Error()
+						errStr = fmt.Sprintf("error with %s: %s", globalNotificationSys.peerClients[index].String(), err.Error())
 					}
 				}()
 			}