Return the correct error from mod_dav_svn if the repository path does not exist.

* subversion/mod_dav_svn/repos.c (get_resource): If the reason for not being
   able to open a repositoy is ENOENT or ENOTDIR, then return a 404 status
   instead of 500.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544259 13f79535-47bb-0310-9956-ffa450edef68