Backporting the following from Apache 2.1-dev to 2.0.49-dev:

  *) Add a hook (insert_error_filter) to allow filters to re-insert
     themselves during processing of error responses. Enable mod_expires
     to use the new hook to include Expires headers in valid error
     responses. This addresses an RFC violation. It fixes PRs 19794,
     24884, and 25123.

Reviewed by: trawick, stoddard


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@102210 13f79535-47bb-0310-9956-ffa450edef68