Custom System Messages provides an alternative to the the System Message Templates for ExpressionEngine. All ExpressionEngine messages, regardless of type or severity, are filtered through the same group of templates, which are only editable within the Control Panel. If you use Git to version your templates, the System Message Templates are left out. On top of this fact the System Message Templates do not not parse other template tags, so embedding other templates, or using tags from third party add-ons do not work... until now.
CSM lets you define which templates to use for your error pages. There are two global options which will be used for all messages, depending on the type, or you can choose specific templates for specific actions, including errors.
For more information please see the documentation.