SMPP failing to retrieve failed message

tl;dr when receiving an `ESME_RUNKNOWNERR` the SMPP transport can fail to lookup the failed message:
```
2014-06-04 12:40:52+0200 [SmppTransceiverProtocol,client] INCOMING << {'header': {'command_status': 'ESME_RUNKNOWNERR', 'command_length': 16, 'sequence_number': 13788510, 'command_id': 'submit_sm_resp'}}
2014-06-04 12:40:52+0200 [VumiRedis,client] Starting factory <HTTPClientFactory: http://syslog:9000/api/2/store/>
2014-06-04 12:40:52+0200 [VumiRedis,client] Could not retrieve failed message: 9998021f1bbd4618aee32569937db316
2014-06-04 12:40:52+0200 [HTTPPageGetter,client] Stopping factory <HTTPClientFactory: http://syslog:9000/api/2/store/>
```

full log here: https://gist.github.com/4738f032bf6c90f6635b
The body of the `submit_sm_resp` PDU (which contains the message identifier) is omitted for unsuccessful responses.

That's unrelated to this check, though -- we get the `message_id` parameter from a call to `get_sequence_number_message_id()` and that will return `None` if we don't have a value in Redis. I'm not entirely sure what can cause that, but we've definitely seen it in production.