There are three error pages in the Nuxeo Platform, each of them can be customized:
nuxeo_error.jsp: This is the default error page
nuxeo_error_security.jsp: It is displayed when a SecurityException is thrown
page_not_found.jsp: It is displayed for an error 404
When the server is deployed, all theses pages are located at server/default/deploy/nuxeo.ear/nuxeo.war/. In the source code, these pages are located at nuxeo-platform-webapp/src/main/resources/nuxeo.war/.
You have to override this nuxeo_error.jsp page in a custom project.
Copy this page inside your custom project in the same relative location as it is in the original Nuxeo sources, that is in a comparable nuxeo.war directory.
Update the parts of the pages you want to customize.
The deployment-fragment.xml of your custom project should contain the following requirement to nicely deploy your customized version of the error pages.