release:publish should not print an error if a package does not exist on npm and it will not be published

[pomek]

📝 Provide detailed reproduction steps (if any)

It's hard to reproduce because you need to find a package that will not be published on npm but it's tracked in the repository.

It happened when I was releasing changes from https://github.com/ckeditor/ckeditor5-package-generator.

✔️ Expected result

Checking "@ckeditor/ckeditor5-package-generator"...

⚠️  Skipping because the package was listed as `options.skipNpmPublish`.

❌ Actual result

Checking "@ckeditor/ckeditor5-package-generator"...
npm ERR! code E404
npm ERR! 404 Not Found - GET https://registry.npmjs.org/@ckeditor%2fckeditor5-package-generator - Not found
npm ERR! 404
npm ERR! 404  '@ckeditor/ckeditor5-package-generator@latest' is not in this registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/pomek/.npm/_logs/2021-10-04T09_53_14_521Z-debug.log

⚠️  Skipping because the package was listed as `options.skipNpmPublish`.

❓ Possible solution

• Execute: https://github.com/ckeditor/ckeditor5-dev/blob/c0ecd4fe678b797167da094a2b09700cf82be64f/packages/ckeditor5-dev-env/lib/release-tools/tasks/releasesubrepositories.js#L351-L356
• Before https://github.com/ckeditor/ckeditor5-dev/blob/c0ecd4fe678b797167da094a2b09700cf82be64f/packages/ckeditor5-dev-env/lib/release-tools/tasks/releasesubrepositories.js#L343.