Consider checking if there's a statusText property. #24
Description
If we're using gateway to potentially ping a server and and see if it's running (example scenario is with Sixpack and confirming it's up and running before executing all the logic for a Sixpack experiment), we may end up with responses that aren't JSON and do not contain a statusText in the response.
https://github.com/DoSomething/gateway-js/blob/master/src/RestApiClient.js#L42
It could be useful to check before passing it to the GatewayError() class, or maybe throwing a simpler error if response isn't JSON. 🤔