fix: report an error whe running site:create nameflag is invalid

[Lxxyx]

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Summary

Fixes #4503

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[danez]

Thanks for your PR.

This looks good to me, only one thing I like to be changed (see inline comment).
Once that is done I can merge.

[github-actions]

📊 Benchmark results

Comparing with 9e1e55b

Package size: 222 MB

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• T (current commit): 222 MB