Is your feature request related to a problem? Please describe.
Yes, similar to #5379, I have a policy that checks if an user already exists. If an user already exists, the execution of the flow is cancelled by means of the Deny Stage. Unfortunately, it is not possible to pass a configurable error message to the Deny Stage. The ak_message function has no effect.

Describe the solution you'd like
I would like to pass an error message to the deny stage to inform the user that an account with a given email address already exist.

Describe alternatives you've considered
I've considered a Prompt Stage to view some text to the user, but I see no way to have a proper control flow between stages.

Additional context
In the code base here there is no error_message passed to stage_invalid().