fix(brownie): throw descriptive error when store not registered

[okwasniewski]

Summary

• Improves error handling when a Brownie store is accessed from JS but not registered on native side
• Throws descriptive error with Swift code example showing how to fix

Changes

When getOrCreateStore() is called for a store that wasn't registered natively, it now throws:

[Brownie] Store "BrownfieldStore" not found. Make sure to register it on the native side before accessing it from JS.

Swift example:
  BrownfieldStore.register(BrownfieldStore(...))

This should be called before React Native starts.

Previously, this would silently return empty objects leading to confusing behavior.