Make CMS directive filtering error be more generic

- Even though the filter method is in the \Magento\Email\Model\Template\Filter class,
  that class is extended by several CMS classes

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

@@ -971,7 +971,7 @@ public function filter($value)
             if ($this->_appState->getMode() == \Magento\Framework\App\State::MODE_DEVELOPER) {
                 $value = sprintf(__('Error filtering template: %s'), $e->getMessage());
             } else {
-                $value = __("We're sorry, an error has occurred while generating this email.");
+                $value = __("We're sorry, an error has occurred while generating this content.");
             }
             $this->_logger->critical($e);
         }