Fix handling of network errors for beforeRetry hook

[sholladay]

Fixes #251
Fixes #275

The PR fixes some problems with hooks caused by how we handle responses:

• Most importantly, it ensures that beforeRetry hooks get called as expected in case of a network error where no response was received. Previously, if a network error occurred, Ky would attempt to clone a non-existent response, causing an exception immediately before the beforeRetry hook, meaning it wouldn't be called. The simplest fix, which I've implemented here, is to just remove the response parameter (which was merely a shortcut) and make people use error.response instead, which IMO is actually a bit clearer anyway, in that it is obvious the response may be undefined. There are some other possible ways to fix this... we could make it a getter, or use error.response && error.response.clone(). Of those two alternatives, I'd prefer a getter, in order to maintain a single source of truth for what the response is.
• Additionally, it ensures that the response received by afterResponse hooks and the error.response from HTTP errors has the custom parseJson function applied to it. I've implemented a _decorateResponse() method to achieve this, which can be used to apply the custom parser anywhere a response is cloned.

[sindresorhus]

Can you update https://github.com/sindresorhus/ky/blob/master/index.d.ts too? Docs-wise.