[sign_in_with_apple] Refine error handling for "password not available case" about dart_packages

[HenriBeck]

I think we can also provide better errors as there is a mapping on the native side: https://developer.apple.com/documentation/authenticationservices/asauthorizationerror/code

[HenriBeck]

@tp After looking more at the whole error handling of the authorization, maybe it also makes sense to only support a single request per authorization on the Dart side.

My reasoning behind this: I didn't find a way of providing both Password and an Apple ID request where I wouldn't run into an error in case no password is stored.

[tp]

My reasoning behind this: I didn't find a way of providing both Password and an Apple ID request where I wouldn't run into an error in case no password is stored.

We can definitely simplify this on the native side (to only allow requesting 1 at a time), so get more logic on the Dart side. I would still allow the consumer to request a list of authentications with a priority/order, so that the usage is very simple.

[HenriBeck]

@tp With the improved API, I think we can close this.

[tp]

@HenriBeck I still think it has 1 ugliness by throwing for a missing Keychain entry instead of returning null. But will verify and attempt a fix for that later. Consumers should definitely execute each login attempt in an individual try/catch.

[HenriBeck]

@tp I think throwing an error is more appropriate here, to be honest. I would consider this a success flow, and as you said, the user should wrap each call in a try/catch.

So I think it's easier to throw an error and not have to handle the null case in the success flow (even with NNDB coming up)