upgrading charms that use symbolic links fails

[James Page]

Most of my charms use symbolic links for the various hooks with a common file for the actual code.

If I try to upgrade a charm I get the following error:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/juju/unit/lifecycle.py", line 73, in run
    self._bundle.extract_to(self._charm_dir)
  File "/usr/lib/python2.7/dist-packages/juju/charm/bundle.py", line 62, in extract_to
    os.symlink(target, os.path.join(directory_path, info.filename))
OSError: [Errno 17] File exists