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successful heal and a no-op since the outdated disks were offline.

cmd/fs-v1_test.go

@@ -303,7 +303,7 @@ func TestFSHealObject(t *testing.T) {
 	defer removeAll(disk)
 
 	obj := initFSObjects(disk, t)
-	err := obj.HealObject("bucket", "object")
+	_, _, err := obj.HealObject("bucket", "object")
 	if err == nil || !isSameType(errorCause(err), NotImplemented{}) {
 		t.Fatalf("Heal Object should return NotImplemented error ")
 	}