Refactor unrescue
Refactor unrescue

- Trap exceptions so its easier to debug if
  there is an exception.
- Add doc string for easier maintenance and
  readability.

Signed-off-by: Chuck Short <753a8074f04ffd62a8fff7516e3ae689be8e35ad@canonical.com>

tpouyer/nova-lxd,tpouyer/nova-lxd