[ISSUE alibaba#11718] Fix ErrorCode have the same code (alibaba#11747)

* [ISSUE alibaba#11718] Fix ErrorCode have the same code

* Fix code style.

api/src/main/java/com/alibaba/nacos/api/model/v2/ErrorCode.java

@@ -178,7 +178,7 @@ public enum ErrorCode {
     /**
      *  node down failure.
      */
-    NODE_DOWN_FAILURE(23001, "node down failure"),
+    NODE_DOWN_FAILURE(23002, "node down failure"),
 
     /**
      *  server error.

api/src/test/java/com/alibaba/nacos/api/model/v2/ErrorCodeTest.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 1999-2021 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.alibaba.nacos.api.model.v2;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import org.junit.Test;
+
+public class ErrorCodeTest {
+    @Test
+    public void testCodeNotSame() {
+        Class<ErrorCode> errorCodeClass = ErrorCode.class;
+
+        ErrorCode[] errorCodes = errorCodeClass.getEnumConstants();
+        Set<Integer> codeSet = new HashSet<Integer>(errorCodes.length);
+
+        for (ErrorCode errorCode : errorCodes) {
+            codeSet.add(errorCode.getCode());
+        }
+
+        assertEquals(errorCodes.length, codeSet.size());
+    }
+}