You can customize the error messages that Caching Proxy returns, and you can define specific messages for particular error conditions. In the Configuration and Administration forms, select Server Configuration -> Error Message Customization. Use this form to select an error condition and specify a particular HTML file to use for that condition.
To customize error messages by editing directives in the proxy configuration file, see the reference section for the directive ErrorPage -- Specify a customized message for a particular error condition.