Fix issue #62

Double check on condition `server_status == 'ERROR'` can be avoid putting the `fail` instruction in the loop

```ruby
timeout(timeout, Errors::Timeout) do
  server_status = 'WAITING'
  until server_status == 'ACTIVE'
    @logger.debug('Waiting for server to be ACTIVE')
    server_status = env[:openstack_client].nova.get_server_details(env, server_id)['status']
    fail Errors::ServerStatusError, server: server_id if server_status == 'ERROR'
    sleep retry_interval
  end
end
```