fixes bug 983294 - BadStatusCodeError doesn't have an error message

@rhelmer r?

The traceback used to look like this:
```
...
  File "/Users/peterbe/dev/MOZILLA/SOCORRO/socorro/webapp-django/crashstats/crashstats/models.py", line 243, in fetch
    raise BadStatusCodeError(resp.status_code, url)
BadStatusCodeError
```

Now it looks like this:

```
...
  File "/Users/peterbe/dev/MOZILLA/SOCORRO/socorro/webapp-django/crashstats/crashstats/models.py", line 244, in fetch
    raise BadStatusCodeError(resp.status_code, url)
BadStatusCodeError: 500: http://localhost:8883/server_status/duration/12
```