Add disksUnavailable healStatus const (#3990)

`disksUnavailable` healStatus constant indicates that a given object
needs healing but one or more of disks requiring heal are offline. This
can be used by admin heal API consumers to distinguish between a
successful heal and a no-op since the outdated disks were offline.

pkg/madmin/examples/heal-object.go

@@ -41,17 +41,17 @@ func main() {
 
 	// Heal object mybucket/myobject - dry run.
 	isDryRun := true
-	err = madmClnt.HealObject("mybucket", "myobject", isDryRun)
+	_, err = madmClnt.HealObject("mybucket", "myobject", isDryRun)
 	if err != nil {
 		log.Fatalln(err)
 	}
 
 	// Heal object mybucket/myobject - this time for real.
 	isDryRun = false
-	err = madmClnt.HealObject("mybucket", "myobject", isDryRun)
+	healResult, err := madmClnt.HealObject("mybucket", "myobject", isDryRun)
 	if err != nil {
 		log.Fatalln(err)
 	}
 
-	log.Println("successfully healed mybucket/myobject")
+	log.Printf("heal result: %#v\n", healResult)
 }

pkg/madmin/examples/heal-objects-list.go

@@ -65,6 +65,8 @@ func main() {
 			switch healInfo := *object.HealObjectInfo; healInfo.Status {
 			case madmin.CanHeal:
 				fmt.Println(object.Key, " can be healed.")
+			case madmin.CanPartiallyHeal:
+				fmt.Println(object.Key, " can't be healed completely, some disks are offline.")
 			case madmin.QuorumUnavailable:
 				fmt.Println(object.Key, " can't be healed until quorum is available.")
 			case madmin.Corrupted:

pkg/madmin/examples/heal-upload.go

@@ -41,17 +41,17 @@ func main() {
 
 	// Heal upload mybucket/myobject/uploadID - dry run.
 	isDryRun := true
-	err = madmClnt.HealUpload("mybucket", "myobject", "myuploadID", isDryRun)
+	_, err = madmClnt.HealUpload("mybucket", "myobject", "myUploadID", isDryRun)
 	if err != nil {
 		log.Fatalln(err)
 	}
 
 	// Heal upload mybucket/myobject/uploadID - this time for real.
 	isDryRun = false
-	err = madmClnt.HealUpload("mybucket", "myobject", "myuploadID", isDryRun)
+	healResult, err := madmClnt.HealUpload("mybucket", "myobject", "myUploadID", isDryRun)
 	if err != nil {
 		log.Fatalln(err)
 	}
 
-	log.Println("successfully healed mybucket/myobject/myuploadID")
+	log.Printf("Heal result for mybucket/myobject/myUploadID: %#v\n", healResult)
 }

pkg/madmin/examples/heal-uploads-list.go

@@ -64,6 +64,8 @@ func main() {
 			switch healInfo := *upload.HealUploadInfo; healInfo.Status {
 			case madmin.CanHeal:
 				fmt.Println(upload.Key, " can be healed.")
+			case madmin.CanPartiallyHeal:
+				fmt.Println(upload.Key, " can be healed partially. Some disks may be offline.")
 			case madmin.QuorumUnavailable:
 				fmt.Println(upload.Key, " can't be healed until quorum is available.")
 			case madmin.Corrupted: