rose.fs_util: improve symlink logic.

Add filename to OSError if target exists.
Don't use os.path.exists for testing existence of symlink. It does not
work on broken link.
Merge pull request #747 from matthewrmshin/symlink.e.filename

rose.fs_util: improve symlink logic.
Merge branch 'master' of github.com:metomi/rose into gh-pages