Change the CacheRoot processing to check for a required

Change the CacheRoot processing to check for a required
value at config time. This saves a lot of wasted processing
if the mod_disk_cache module is loaded but no CacheRoot
was provided. This fix also adds code to log an error
and avoid useless pallocs and procesing when the computed
cache file name cannot be opened. This also updates the
docs accordingly.  [Paul J. Reder]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97291 13f79535-47bb-0310-9956-ffa450edef68