Add support for tier-remove and tier-verify (#14382)

* Add tier remove support only if it's empty
* Add support for tier verify

cmd/gateway-common.go

@@ -335,6 +335,8 @@ func ErrorRespToObjectError(err error, params ...string) error {
 	switch minioErr.StatusCode {
 	case http.StatusMethodNotAllowed:
 		err = toObjectErr(errMethodNotAllowed, bucket, object)
+	case http.StatusBadGateway:
+		return BackendDown{Err: err.Error()}
 	}
 	return err
 }