Improved grabService error message (#20)

* Update Symfony.php

Adding hint about unused services not being available, see Codeception/Codeception#5828 and https://stackoverflow.com/a/63131476/1668200

* Update Symfony.php

Switching to *test* config, see Codeception/Codeception#5828 (comment)

Author: ThomasLandauer

src/Codeception/Module/Symfony.php

@@ -574,7 +574,9 @@ public function grabService($service)
     {
         $container = $this->_getContainer();
         if (!$container->has($service)) {
-            $this->fail("Service $service is not available in container");
+            $this->fail("Service $service is not available in container.
+            If the service isn't injected anywhere in your app, you need to set it to `public` in your `config/services_test.php`/`.yaml`,
+            see https://symfony.com/doc/current/testing.html#accessing-the-container");
         }
         return $container->get($service);
     }