Raise error for invalid relay (#2128)

* Raise error for invalid relay

* Keep error messages the same

* Distinguish this from validation failed

dnscrypt-proxy/serversInfo.go

@@ -467,8 +467,9 @@ func route(proxy *Proxy, name string, serverProto stamps.StampProtoType) (*Relay
 		}
 	}
 	if len(relayStamps) == 0 {
-		dlog.Warnf("Empty relay set for [%v]", name)
-		return nil, nil
+		err := fmt.Errorf("Non-existent relay set for server [%v]", name)
+		dlog.Warn(err)
+		return nil, err
 	}
 	var relayCandidateStamp *stamps.ServerStamp
 	if !wildcard || len(relayStamps) == 1 {