Clarify missing css error message.

 * Avoid confusion by quoting the error.ref. Without quotes an error message such "(file) contains a reference to the file path." is possible.

lib/friendly-errors/formatters/missing-css-file.js

@@ -22,7 +22,7 @@ function formatErrors(errors) {
         chalk.red('Module build failed: Module not found:')
     );
     for (let error of errors) {
-        messages.push(`${error.file} contains a reference to the file ${error.ref}.`);
+        messages.push(`${error.file} contains a reference to the file "${error.ref}".`);
         messages.push('This file can not be found, please check it for typos or update it if the file got moved.');
         messages.push('');
     }