process_input() to skip the step. The model is not called, the reason is logged once
per change rather than once per step, and the runtime asks again shortly.
Idle is the old name for this class. It still imports, with a deprecation warning.ApplicationError
Raise from process_input() to skip a step.
Raise from
Subclass it when a reason should be distinguishable in code or in logs. The loop catches the base
class, so every subclass skips the step the same way.