Bad URL on ExperimentClient.start/fetch

We've noticed that we seem to be getting a fair few errors whenever we call the `ExperimentClient.start` / `ExperimentClient.fetch` methods.

We use Sentry for our error reporting, and we have set it up so that we send most errors through to Sentry whenever we get an error from the start call. You can see the exact details/numbers here via our Sentry reporting:

* Start - https://trustpilot.sentry.io/share/issue/82f376794f074c72bfa7ca184b23de33/
* Fetch - https://trustpilot.sentry.io/share/issue/c0fe8af82ba84cd0a1c4a3d85f830122/

Note that the error name contains the error code/domain


Based on the error code we're getting in the callbacks, it seems like the error is due to a bad URL ([see NSURLErrorDomain codes](https://gist.github.com/samirahmed/ee52a076dd19966c4c9d)), though based on the breadcrumbs in Sentry, the url seems to be fine.