Let gpstop error message about environment variable readable

[water32]

When the environment variables COORDINATOR_DATA_DIRECTORY are not set correctly, the error message is meaningless

Fixes #ISSUE_Number

What does this PR do?

Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change (fix or feature with breaking changes)
• Documentation update

Breaking Changes

Test Plan

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• Requested review from cloudberry committers

Additional Context

CI Skip Instructions

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[github-actions]

Hi, @water32 welcome!🎊 Thanks for taking the effort to make our project better! 🙌 Keep making such awesome contributions!

[tuhaihe]

Hi @water32 thanks for your contribution! Could you please update the commit title and message body following our commit convention .gitmessage for a better context? Thanks.

[tuhaihe]

Hi @water32 welcome to rebase your commit. You can take the following commit message title and commit body as a reference:

Improve error message when COORDINATOR_DATA_DIRECTORY is unset

This commit updates gpstop and gpstart to use shared logic when
determining the coordinator data directory.

By relying on the common helper, users receive clearer and more
actionable error messages when COORDINATOR_DATA_DIRECTORY is unset
or invalid, making startup and shutdown failures easier to diagnose.