The check of the overlays

Expresso.VerifierTransformer runs it after the other transformers, one time for each deck module at compile time, so an error stops the compile. It calls Expresso.Overlay.Check.slide/1 for each slide. An error from the check becomes a Spark.Error.DslError with the path of the slide. It then calls Expresso.Overlay.Properties.registered_state/2, which refuses a state that takes the name of a property that the theme registers.