HDFS-16717. Replace NPE with IOException in DataNode.class

[ZanderXu]

Description of PR

In current logic, if storage not yet initialized, it will throw a NPE in DataNode.class. Developers or SREs are very sensitive to NPE, so I feel that we can use IOException instead of NPE when storage not yet initialized.

[hadoop-yetus]

:broken_heart: -1 overall

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		564m 33s

Reason	Tests
Failed junit tests	hadoop.hdfs.TestSnapshotCommands

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4699/1/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/4699
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versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[hadoop-yetus]

:broken_heart: -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	1m 8s		Docker mode activated.
			_ Prechecks _
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			_ trunk Compile Tests _
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		566m 11s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4699/2/artifact/out/Dockerfile
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modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
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versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[ZanderXu]

@Hexiaoqiao @ayushtkn Masters, can help me review this patch? Thanks

[ZanderXu]

@Hexiaoqiao Master, ping. Can help me review this patch? Thanks

[hadoop-yetus]

:broken_heart: -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	0m 49s		Docker mode activated.
			_ Prechecks _
+1 :green_heart:	dupname	0m 0s		No case conflicting files found.
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			_ trunk Compile Tests _
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		466m 3s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4699/3/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/4699
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uname	Linux 33d5e27962a4 4.15.0-175-generic #184-Ubuntu SMP Thu Mar 24 17:48:36 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
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Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4699/3/testReport/
Max. process+thread count	2042 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4699/3/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[Hexiaoqiao]

Committed to trunk. Thanks @ZanderXu for your contributions!

[steveloughran]

just want to say this was a really amusing title in the git logs. might use it myself "replace NPE with working code in ..."