Eliminate another instance of errors being missused as part of the
standard control flow.  Again, this not only improves ST performance
but MT speed / scalability even more so.

* subversion/libsvn_fs_fs/tree.c
  (open_path_flags_t): Define flag that lets open_path return a
                       NULL result instead of an error.
  (open_path): Extend implementation accordingly.
  (fs_closest_copy): Use open_path with the new flag and we can
                     remove the previous path check which had the
                     internal error -> special result conversion.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586953 13f79535-47bb-0310-9956-ffa450edef68