certbot: Update storage.get_link_target (#4750) (#4923)
certbot: Update storage.get_link_target (#4750) (#4923)

* certbot: Update storage.get_link_target (#4750)

* The `get_link_target` function raises `errors.CertStorageError` when
  link does not exists.

* certbot: Fix typo in storage.get_link_target.

stweil/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt