The default error page is:
Enterprise customers may upload a custom error page by contacting Incapsula support. As you can see in the screenshot, "error code" and "description" will change as per different scenarios - blocking or site unavailable - but there is provision for only single error page (background). Unfortunately it is not possible to customize the web page for different scenarios. Also, the custom error page will be applied at account level - meaning it will be applicable to all the sites under a particular account. To know more, refer document 'Custom Error Pages'.
Additional Detail - In the above screenshot Error code 15 represents a block event due to a security rule. To know more about Incapsula error codes, this document can be referred.