ZEN-6475: show device description for deferred object internal error

DEMO - snippet from zencommand log:
```
2014-10-21 18:25:23,656 ERROR zen.zencommand: Testing Internal error where deferred object not in dictionary for root:*****@10.175.210.226:22. Command = '/usr/bin/uptime' Data = ' 09:11:13 up 74 days, 22:01,  0 users,  load average: 0.00, 0.00, 0.00
' Code = '0' Stderr = ''
```