fix bug 1020487 - add sentry to kumascript via raven

**DO NOT MERGE**

I need help finding a way to spot-check that errors are being sent up to [errormill](https://errormill.mozilla.org/mdn/mdn-dev/).

In theory, to enable sentry error reporting, add:
```
"server": {
  ...
  "sentry_dsn": "<sentry DSN from https://mana.mozilla.org/wiki/display/websites/developer.mozilla.org+Cluster#developer.mozilla.orgCluster-Dev",
  ...
}
```
to your `kumscript_settings_local.json` and restart kumascript.