[selfupdate] Fix an issue when 'jiri update' complains cannot update.

When latest prebuilt for jiri is not yet available, running 'jiri
update' will print an error message indicating cannot selupdate
jiri. This change fixes this issue.

Bug: 37581
Change-Id: I7cdef0bc88e64046597e9a64e5161d0e757d6bb9
diff --git a/update.go b/update.go
index 6a6597b..6c043b6 100644
--- a/update.go
+++ b/update.go
@@ -49,6 +49,9 @@
 		// CIPD HTTP endpoint does not allow HTTP HEAD.
 		// Download the Jiri archive directly.
 		b, err := downloadBinary(JiriCIPDEndPoint, commit)
+		if err == updateNotAvailableErr {
+			return err
+		}
 		if err != nil {
 			return fmt.Errorf("cannot download latest jiri binary, %s", err)
 		}
@@ -93,9 +96,8 @@
 		if err != updateNotAvailableErr && err != updateVersionErr &&
 			err != updateTestVersionErr {
 			return err
-		} else {
-			return nil
 		}
+		return nil
 	}
 
 	args := []string{}