Return a little bit more error information when, say a disk is full or...

Return a little bit more error information when, say a disk is full or something gets write protected. Note that in some cases mod_cache.c will_also_ log a 'cache: store_headers failed' subsequently.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@620489 13f79535-47bb-0310-9956-ffa450edef68