Improve the error msg for private catalogsource validating

[YCShen1010]

What this PR does / why we need it: When running the setup_singleton.sh with -- enable-private-catalog flag set, the error message of CatalogSource validation has been updated

Improvement：

• When the --enable-private-catalog flag is set, there is a warning message that prompts the namespaces of singleton operators should be in the same namespace as the singleton CatalogSource.
• If the specific CatalogSource in its namespace is not found in the cluster, then the script will try to get an available global CatalogSource and will error out if there is more than one CatalogSource available in the entire cluster.

Which issue(s) this PR fixes: Issue: https://github.ibm.com/IBMPrivateCloud/roadmap/issues/63998#issuecomment-83385625

[ibm-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Daniel-Fan, YCShen1010

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