Custom `handleResponse` Not Called on Fetch Failure

Firstly, thanks for the awesome library! I've been switching to React from Angular and it's starting to grow on me :smile:

I'm trying to inject a custom error message to any fetch that fails without a more specific error message so I don't need to update all my components individually.

Looking at [the code](https://github.com/heroku/react-refetch/blob/master/src/components/connect.js#L292), it seems that `handleResponse` is called as a `.then()`, meaning if the `fetch` returns a failed `Promise` (eg: can't connect to foo.site.com), I have no way of injecting a custom json failure message.

Do you have any suggestions of a workaround? Perhaps `handleReponse` could also be called on failure?

Thoughts?