fix exit status code (fabiolb#637)

Fabio always returned status code 1. Event on clean exit.
Shutdown of http server is treated as fatal:
 [FATAL] http: Server closed
 [FATAL] ui: http: Server closed

Http server Serve method always returns non nil error:
https://golang.org/pkg/net/http/#Server.Serve

ErrServerClosed should be treated as clean exit.
"After Shutdown or Close, the returned error is ErrServerClosed."

proxy/serve.go

@@ -103,5 +103,8 @@ func serve(ln net.Listener, srv Server) error {
 	mu.Lock()
 	servers = append(servers, srv)
 	mu.Unlock()
-	return srv.Serve(ln)
+	if err := srv.Serve(ln); err != http.ErrServerClosed {
+		return err
+	}
+	return nil
 }