YARN-11893. Extend REST API error messages with exception classes

[p-szucs]

Change-Id: I7c08bd438fe17dfed8641f6883d8dbc9566fc771

Description of PR

In GenericExceptionHandler's toResponse method we put only the exception message into the RemoteExceptionData object.

It would be a bit more informative if we would use throwable.toString() instead of throwable.getMessage() as the former extends the error message with the exception class as well, similarly to previous versions.

How was this patch tested?

Tested on a Hadoop cluster

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[K0K0V0K]

Thanks @p-szucs for the fix! Seems good to me. If you can add some basic unit test as well, that would be great!

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
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+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	33m 5s		trunk passed
+1 💚	compile	0m 46s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
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+1 💚	shadedclient	27m 59s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 40s		the patch passed
+1 💚	compile	0m 38s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javac	0m 38s		the patch passed
+1 💚	compile	0m 37s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	javac	0m 37s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 24s		the patch passed
+1 💚	mvnsite	0m 41s		the patch passed
+1 💚	javadoc	0m 44s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javadoc	0m 41s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	spotbugs	2m 40s		the patch passed
+1 💚	shadedclient	27m 36s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	5m 28s		hadoop-yarn-common in the patch passed.
+1 💚	asflicense	0m 35s		The patch does not generate ASF License warnings.
		130m 1s

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Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8066/1/artifact/out/Dockerfile
GITHUB PR	#8066
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[slfan1989]

@p-szucs Thanks for the contribution! Since some changes touch the MapReduce module, I suggest splitting this into two PRs.