OfferingsManager: expose underlying error when ProductsManager returns an error

[NachoSoto]

This will help figure out the reason behind #1765.
We were currently ignoring the error returned by ProductsManager.

In the example below, we can see the underlying issue is SKErrorDomain code 5.

Before:

Error fetching offerings - The operation couldn’t be completed. (RevenueCat.OfferingsManager.Error error 1.)

After:

Error fetching offerings - The operation couldn’t be completed. (RevenueCat.OfferingsManager.Error error 1.)
Underlying error: The operation couldn’t be completed. (SKErrorDomain error 5.)
ERROR: 😿‼️ There is an issue with your configuration. Check the underlying error for more details. There's a problem with your configuration. None of the products registered in the RevenueCat dashboard could be fetched from App Store Connect (or the StoreKit Configuration file if one is being used).
More information: https://rev.cat/why-are-offerings-empty

[taquitos]

Yesss, this is awesome. 🎈🐐