Make some 'ra_dav' commit error messages for out of date cases match those
of the generic libsvn_repos commit logic. (See commit.c out_of_date())

* subversion/mod_dav_svn/repos.c
  (do_out_of_date_check,
    remove_resource): Select error message based on node type, like in the
    libsvn_repos commit logic.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558247 13f79535-47bb-0310-9956-ffa450edef68