Check if our checker can be used before enforcing it.

The buffer could be in some derived mode that flycheck wont activate
the checker for, which results in errors if we try to enforce it.

Author: xendk

drupal/flycheck.el

@@ -53,7 +53,9 @@ so no need to highlight it twice."
     (set (make-local-variable 'flycheck-phpcs-standard) drupal/phpcs-standard)
 
     (when drupal/flycheck-phpcs-js-and-css
-      (if (apply 'derived-mode-p (append drupal-css-modes drupal-js-modes))
+      (if (and (apply 'derived-mode-p (append drupal-css-modes drupal-js-modes))
+               (flycheck-may-use-checker 'css-js-phpcs)
+               )
           (set (make-local-variable 'flycheck-checker) 'css-js-phpcs)
         )
       )