Add connector id to the registration error message

Right now it is not clear what connector is failing. It will be easier to debug with more specific error message.

Related to #400.

cmd/dex-worker/main.go

@@ -191,7 +191,7 @@ func main() {
 	for _, cfg := range cfgs {
 		cfg := cfg
 		if err = srv.AddConnector(cfg); err != nil {
-			log.Fatalf("Failed registering connector: %v", err)
+			log.Fatalf("Failed registering connector '%s': %v", cfg.ConnectorID(), err)
 		}
 	}