aperture+challenger: add error channel to challenger

To make sure we can capture errors in the challenger's invoice
subscription, we hand the main error channel to the challenger so it can
report back errors on it.

aperture.go

@@ -110,7 +110,10 @@ func run() error {
 			Value: price,
 		}, nil
 	}
-	challenger, err := NewLndChallenger(cfg.Authenticator, genInvoiceReq)
+	errChan := make(chan error)
+	challenger, err := NewLndChallenger(
+		cfg.Authenticator, genInvoiceReq, errChan,
+	)
 	if err != nil {
 		return err
 	}
@@ -164,7 +167,6 @@ func run() error {
 	)
 	log.Infof("Starting the server, listening on %s.", cfg.ListenAddr)
 
-	errChan := make(chan error)
 	wg.Add(1)
 	go func() {
 		defer wg.Done()