Service crash handling

A few little improvements. ;)

---

Currently https://counterwallet.co/_t_api is saying:

```json
{"counterpartyd": "NOT OK", "counterblockd_ver": "1.4.0", "counterpartyd_ver": "?", "counterblockd": "OK", "counterblockd_check_elapsed": 0.0035178661346435547, "counterpartyd_last_block": "?", "local_online_users": 0, "counterpartyd_check_elapsed": 0.0005960464477539062, "counterblockd_error": null, "counterpartyd_last_message_index": "?"}
```

It makes the library crash before it gets to the point where the balances are listed to the user. Sad. But hey! this pull request fixes it!
Now, when statusCode is not right (`!= 200`) we turn things into exception which is later catched and printed to the user. :)