Implement error handling in BuyerExchangeActor

**Description** Implement a simple (fast-fail) error handling mechanism for the BuyerExchangeActor.
**Testing** Added UTs for the failure cases
**Coverage** Here are the affected classes:

- BuyerExchangeActor: 100%
- DefaultExchange: 90,48%
- InitializedBuyerExchange: 100%