Previously, a non-ok http response would reject with the response
object. It's better for rejections to be of type Error so that the
full stack trace information is available; plus, the TypeScript type
definition assumes that the error object is always instanceof Error.

Instead, failed responses reject with a FetchError, with the
underlying Response object available as error.response.

This is a backward-incompatible change: users who expected `error` to
be of type Response now have to refer to `error.response` instead.

This is necessary for TypeScript code to be permitted to use
FetchError as a value at runtime, e.g. to perform an `instanceof
FetchError` check.