error handling: Throw the whole response on fetch error
error handling: Throw the whole response on fetch error

* handle both response error and error in json the same way
* do throw the whole response so the catcher has access to all fields
* verified, no changed to any other code needed, as currently none
of the try-catch-ers of fetch calls care about the error object

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile