Fix Olark post-lead capture

https://github.com/gocardless/splash-pages/pull/68 disabled the opening of an Olark window when a large prospect goes through lead capture. This re-enables it in a nicer way.
This used to be:

```js
var prospect;
try {
  var prospect = data.prospect;
} catch (e) {}
```

Is this going to cause problems?