Report an ArtifactError when `allowedUnsafeExecutions` is not allowlisted

[jamietanna]

Via #43939, we should report an Artifact Error when a repository user intends to use an "unsafe" option.

This would surface it more visibly, and allow repository owners to know when something they're potentially expecting to see run is instead being skipped.

This should also introduce a self-hosted config option, reportUnsafeExecutionsAsArtifactError to disable this. This will be on-by-default.