HDFS-15043. RBF: The detail of the Exception is not shown in ZKDelegationTokenSecretManagerImpl (#1748)

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/router/security/token/ZKDelegationTokenSecretManagerImpl.java

@@ -44,7 +44,7 @@ public class ZKDelegationTokenSecretManagerImpl extends
     try {
       super.startThreads();
     } catch (IOException e) {
-      LOG.error("Error starting threads for zkDelegationTokens ");
+      LOG.error("Error starting threads for zkDelegationTokens", e);
     }
     LOG.info("Zookeeper delegation token secret manager instantiated");
   }