Extend AppExit::Error(NonZero<u8>) to AppExit::Error(NonZero<u8>, Option<String>))

## What problem does this solve or what need does it fill?

An `u8` often cannot describe the error well.

It will be helpfull if allowing to insert an `Option' for error message.

Especially for who need to call `update()` and handle error manually.

## What solution would you like?

Extend `AppExit::Error(NonZero)` to `AppExit::Error(NonZero, Option))`

## What alternative(s) have you considered?

Add a new `AppExit::Error(String)`.

Start by searching for AppExit::Error and the update() call sites to understand how the existing variant is constructed and handled. Review the issue discussion before choosing between the requested optional message and the proposed String variant; done means the selected API supports descriptive errors without breaking or overlooking existing handling.

Written by the indexing model from the issue text.