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

• [x] 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

---

[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.