Add method / status for 'Bad Gateway' (502) or 'Service Unavailable' (503)

[AKlaus]

Use-case:

1. An infrastructure logic calls an external service that fails, so the consumer has to receive 502 'Bad Gateway' or 503 'Service Unavailable' HTTP status code (see this SO discussion on the preferred HTTP code).
2. The library containing the infrastructure code doesn't have a dependency on Microsoft.AspNetCore.App, so returning IDomainResult would be handy.

Task

Add additional extension method along with DomainOperationStatus state:

• ErrorServiceUnavailable that gets automatically converted to HTTP code 503 'Service Unavailable' .