The upgrade code will automatically create a new page template,
error_default. This will be used to display error pages. The new tags
<txp:error_status /> and
<txp:error_message />, when placed in the template, will include the error code (“404 Not Found”) and message respectively.
If a template matching the error code is found (“error_404”, etc), that will be used as the template.
error_default is used when no matching template exists. If
error_default is missing, or if the database is unavailable for some reason, a hard-coded page will be used as a last chance fallback.
The code is in the 4.0 development branch, and will be included in the next release (4.0.2).