[hotfix][typos] Improve exception message of item operator

[wangxlong]

What is the purpose of the change

Improve exception of item operator when not supporting.

Brief change log

change exception message from 'Expect an array or a map' to 'Expect an array, a map or a row'

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit 8c088e9 (Fri Feb 19 07:30:18 UTC 2021)

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CI report:

• 8c088e9 Azure: SUCCESS

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[wangxlong]

Hi @libenchao, could you help to have a review, thanks~

[libenchao]

@wangxlong Thanks for your contribution, LGTM, merging..