* Consequently use disk_cache: in error log strings

PR: 39380
Submitted by: Niklas Edmundsson <nikke acc.umu.se>
Reviewed by: rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@396252 13f79535-47bb-0310-9956-ffa450edef68