We should not ascribe all problems detected by is_changed_node()
to simple out-of-date-ness.  FS corruption etc. should still
reported as such to (ultimately) the client.

* subversion/libsvn_fs_fs/tree.c
  (copy_helper): only report "node not found" as "out of date"

Found by: ivan

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527084 13f79535-47bb-0310-9956-ffa450edef68