Error out on configurations where the cache provider starts with a '/'.

Saves certain people a bit of time figuring out why their caching
doesn't work...

* modules/cache/mod_cache.c

  (add_cache_enable): Throw an error when a provider starting with
   a '/' is encountered.  The user probably switched arguments.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@151232 13f79535-47bb-0310-9956-ffa450edef68