[Config] Add deprecation warning

If someone extends FileLoader and do not call the parent constructor a
deprecation warning should be triggered.

Author: dosten

Loader/FileLoader.php

@@ -86,7 +86,12 @@ public function import($resource, $type = null, $ignoreErrors = false, $sourceRe
                 // we fallback to the current locator to keep BC
                 // as some some loaders do not call the parent __construct()
                 // @deprecated should be removed in 3.0
-                $locator = $loader->getLocator() ?: $this->locator;
+                $locator = $loader->getLocator();
+                if (null === $locator) {
+                    @trigger_error('Not calling the parent constructor in '.get_class($loader).' which extends '.__CLASS__.' is deprecated since version 2.7 and will not be supported anymore in 3.0.', E_USER_DEPRECATED);
+                    $locator = $this->locator;
+                }
+
                 $resource = $locator->locate($resource, $this->currentDir, false);
             }