[in_app_purchase] Fix error message when trying to consume purchase on iOS

[mvanbeusekom]

Redo of #3750, now with correct version number.

Fixes the error message when trying to consume a "consumable product" on iOS. On iOS consuming products manually is not supported and therefore the consumePurchase method on iOS will result in a UnsupportedError. Unfortunately the error message of the error mentioned the "Android" platform:

consume purchase is not available on Android

This PR updates the error message and also extended the unit test to ensure the correct platform is mentioned.

The problem is mentioned as part of issue flutter/flutter#60763 (see this comment)

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[BeMacized]

LGTM!