Merge r1506474 from trunk:

* server/core.c (set_document_root): Improve error message for
  inaccessible docroot.

Submitted by: jorton
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1747057 13f79535-47bb-0310-9956-ffa450edef68