[Major 3.0.0] Improve error handling (HTTP + Bridge events)

[kiftio]

What are you trying to accomplish?

Includes changes from https://github.com/Shopify/checkout-sheet-kit-android/pull/89

Decode error events that are send over the bridge, and return an appropriate error in the CheckoutEventProcessor's onCheckoutFailed() callback.

This is part 1 of the graceful degradation improvements we're making. Part 2 will include a web fallback in the event that errors are related to Checkout Sheet Kit code.

Before you deploy

• [x] I have added tests to support my implementation
• [x] I have read and agree with the contributing documentation
• [x] I have read and agree with the code of conduct documentation
• [x] I have updated the README (if applicable).