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
This happens before we start sending the message parts. Now that you mention it, though, we should explicitly delete any existing value in case we're retrying a message we've already sent or something.