Improving the "required flag(s) "%s" not set" error message by not printing "(s)" if a single required flag is missing

[nestoroprysk]

Improving the "required flag(s) "%s" not set" error message by not printing "(s)" if a single required flag is missing.

If a single flag is missing, the error message is of the form "required flag "%s" not set".

Otherwise, the error message remains unchanged.

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[nestoroprysk]

signed in the agreement

[jharshman]

@nestoroprysk please add a description to your PR.
Add details that will help maintainers get on the same page as you about this change.

[nestoroprysk]

@jharshman updated the description and resolved conflicts

[github-actions]

This PR is being marked as stale due to a long period of inactivity

[marckhouzam]

I think this makes sense but needs a proper review. Re-opening.