Provide improved error messages when entering a promo code fails.
There are three system configuration options under Sales -> Checkout -> Promo Code Error Messages. You can display additional information about the error (for example, if the customer is not in the correct group for the rule, you can display the required groups). In addition, error messaging for the more complex condition-based and action-based rules can be turned off.
Translation file available for English and German (partial).
Each message is wrapped in a class to use for styling:
promo_error_message: Wraps entire message.
promo_error_additional: Wraps additional information if enabled in System Config.
promo_error_heading: Wraps heading for aggregated messages (i.e., The following conditions must be met:).
promo_error_item: Used in conjunction with promo_error_heading; wraps each condition required for the promo code.
There is a CSS file that will indent nested messages included.