MC-15575: Correct filter output

Author: Joan He

app/code/Magento/Email/Model/Template/Filter.php

@@ -849,7 +849,7 @@ public function cssDirective($construction)
             return $css;
         } else {
             // Return CSS comment for debugging purposes
-            return '/* ' . sprintf(__('Contents of %s could not be loaded or is empty'), $file) . ' */';
+            return '/* ' . __('Contents of the specified CSS file could not be loaded or is empty') . ' */';
         }
     }
 

dev/tests/integration/testsuite/Magento/Email/Model/Template/FilterTest.php

@@ -267,7 +267,7 @@ public function cssDirectiveDataProvider()
             'Empty or missing file' => [
                 TemplateTypesInterface::TYPE_HTML,
                 'file="css/non-existent-file.css"',
-                '/* Contents of css/non-existent-file.css could not be loaded or is empty */'
+                '/* Contents of the specified CSS file could not be loaded or is empty */'
             ],
             'File with compilation error results in error message' => [
                 TemplateTypesInterface::TYPE_HTML,