signing workflow produces no useful error for wrong artifact category

If you create a signing workflow and supply an artifact of category debian:binary-package rather than debian:binary-packages, the workflow is stuck in state "Running" without creating any work requests nor giving any indication what went wrong. It is only when you dig into the celery log that you see Workflow orchestrator failed to pre-process task data for WorkRequest and an actual error.

• After a failure to process dynamic data, the workflow should be aborted rather than stuck in running.
• The processing failure should be communicated to the user by some means.