Function emit_state_with_error 

pub fn emit_state_with_error<E: EventEmitter>(
    channel: &E,
    request_id: &str,
    state: TransferState,
    error: Option<AppError>,
) -> Result<(), AppError>

Emit a transfer:state event with an optional AppError attached.

Intended for Failed transitions: pass the underlying error so the frontend’s applyStateEvent can hydrate the failure reason onto the transfer record. Non-Failed callers should keep using emit_state.