Fix missing error handling in WriteUsersInPostgreSQL

[balinorLiang]

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• New feature
• Bug fix
• Documentation
• Testing enhancement
• Other

Other Information:
It seems that the error returned by stdout, stderr, err := exec.Exec(...) could be overwritten by err = WriteUsersSchemasInPostgreSQL(ctx, exec, users), so this patch addresses that issue.

[tjmoore4]

Thanks for the submission, @balinorLiang!