refactor: XHR errors have response/code fields

So that there's no chance of data loss and it's trivial to get the response code (because it's hella useful)

Fixes #1866
Fixes #1876

request/request.js

@@ -93,7 +93,8 @@ module.exports = function($window, Promise) {
 						}
 						else {
 							var error = new Error(xhr.responseText)
-							for (var key in response) error[key] = response[key]
+							error.code = xhr.status
+							error.response = response
 							reject(error)
 						}
 					}