When connecting to a WiFi proxy server, SwiftyStoreKit prompts "bag load failed" when initiating payment. Can you list this exception as an error enumeration? Use unified text prompts

## Platform
- [ ] iOS
- [ ] macOS
- [ ] tvOS

### In-app purchase type
- [ ] Consumable
- [ ] Non-consumable
- [ ] Auto-Renewable Subscription
- [ ] Non-Renewing Subscription

### Environment
- [ ] Sandbox
- [ ] Production

### Version
*Please replace this with the version of SwiftyStoreKit you're using.*
0.15.0
### Related issues
*Please replace this with references to similar issues (either open or already closed).*

## Report

### Issue summary
*Please replace this with an explanation of the issue, including reproduction steps.*

### What did you expect to happen
*Please replace this with what you expected to happen.*

### What happened instead
*Please replace this with what happened instead.*