[CELEBORN-1420] Fix mapreduce job will throw an exit exception after it succeeded

### What changes were proposed in this pull request?
Set the correct flag to remove a redundant exception.

### Why are the changes needed?
To remove a redundant exception.

### Does this PR introduce _any_ user-facing change?
NO.

### How was this patch tested?
Manually test.

Closes #2505 from FMX/b1420.

Authored-by: mingji <fengmingxiao.fmx@alibaba-inc.com>
Signed-off-by: mingji <fengmingxiao.fmx@alibaba-inc.com>

Author: FMX

client-mr/mr/src/main/java/org/apache/celeborn/mapreduce/v2/app/MRAppMasterWithCeleborn.java

@@ -18,6 +18,7 @@
 package org.apache.celeborn.mapreduce.v2.app;
 
 import java.io.IOException;
+import java.lang.reflect.Field;
 
 import org.apache.hadoop.fs.FSDataOutputStream;
 import org.apache.hadoop.fs.FileStatus;
@@ -149,6 +150,17 @@ public static void main(String[] args) {
               Integer.parseInt(nodeHttpPortString),
               appSubmitTime,
               rmAppConf);
+
+      // set this flag to avoid exit exception
+      try {
+        Field field = MRAppMaster.class.getDeclaredField("mainStarted");
+        field.setAccessible(true);
+        field.setBoolean(null, true);
+        field.setAccessible(false);
+      } catch (NoSuchFieldException e) {
+        // ignore it for compatibility
+      }
+
       ShutdownHookManager.get()
           .addShutdownHook(
               () -> {