add some String() to make codes easier to read and debug

go/operation/lookup.go

@@ -3,7 +3,7 @@ package operation
 import (
 	"encoding/json"
 	"errors"
-	_ "fmt"
+	"fmt"
 	"math/rand"
 	"net/url"
 	"strings"
@@ -22,6 +22,10 @@ type LookupResult struct {
 	Error     string     `json:"error,omitempty"`
 }
 
+func (lr *LookupResult) String() string {
+	return fmt.Sprintf("VolumeId:%s, Locations:%v, Error:%s", lr.VolumeId, lr.Locations, lr.Error)
+}
+
 var (
 	vc VidCache
 )