HDFS-16723. Replace incorrect SafeModeException with StandbyException in RouterRpcServer.class

[ZanderXu]

Description of PR

Incorrect code as below:

/**
 * ...
 * @throws SafeModeException If the Router is in safe mode and cannot serve
 *                           client requests.
 */
void checkOperation(OperationCategory op)
    throws StandbyException {
  ...
}

I feel that we should fix the incorrect SafeModeException and use StandbyException instead of it.

[ayushtkn]

LGTM

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 56s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell was not available.
+0 🆗	detsecrets	0m 1s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	38m 21s		trunk passed
+1 💚	compile	0m 58s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	compile	0m 55s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	0m 51s		trunk passed
+1 💚	mvnsite	1m 3s		trunk passed
+1 💚	javadoc	1m 8s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	1m 16s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	1m 47s		trunk passed
+1 💚	shadedclient	21m 13s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 42s		the patch passed
+1 💚	compile	0m 42s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javac	0m 42s		the patch passed
+1 💚	compile	0m 38s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	0m 38s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 25s		the patch passed
+1 💚	mvnsite	0m 40s		the patch passed
+1 💚	javadoc	0m 41s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	0m 57s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	1m 27s		the patch passed
+1 💚	shadedclient	20m 39s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	35m 40s		hadoop-hdfs-rbf in the patch passed.
+1 💚	asflicense	0m 53s		The patch does not generate ASF License warnings.
		133m 42s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4716/1/artifact/out/Dockerfile
GITHUB PR	#4716
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 1e5c4b974365 4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / d20cc5c
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4716/1/testReport/
Max. process+thread count	2154 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs-rbf U: hadoop-hdfs-project/hadoop-hdfs-rbf
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4716/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[ZanderXu]

@ayushtkn Master, Thank you very much for helping me review this patch.