[FLINK-18500][table] Make the legacy planner exception more clear whe…

[danny0405]

…n resolving computed columns types for schema

What is the purpose of the change

The original stack trace throws an UnsupportedOperationException with null error message which is very confusing.
The new message suggest user to use the new planner.

Brief change log

• Enhance the error message for computed columns of legacy planner
• Add test cases

Verifying this change

Added UT.

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

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Automated Checks

Last check on commit 027c4cb (Wed Jul 22 08:55:23 UTC 2020)

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[godfreyhe]

Thanks for the improvement, LGTM