pr-upload: fix --warn-on-upload-failure functionality

[mislav]

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To reproduce: brew pr-upload --no-commit --warn-on-upload-failure in a directory with a few bottles where at least one is already uploaded to GitHub Packages.

For bottle uploads that would be skipped due to preupload_check warning about skipped uploads and returning early, the old code errored out due to JSON schema validation failure since the upload was allowed to resume:

"error"=>"value at /manifests/0/annotations/org.opencontainers.image.ref.name is not a string"

This change ensures that the upload is actually skipped if preupload_check returns no result.