Reject promise if network request failed

Summary:This adds missing check and promise reject for failing network requests. Which was missing in the fetch.js update. Sorry for the trouble.

cc davidaurelio
Closes https://github.com/facebook/react-native/pull/6368

Differential Revision: D3026830

Pulled By: davidaurelio

fb-gh-sync-id: 028037678e148bb9b0a6a098d29ec7d82f64b607
shipit-source-id: 028037678e148bb9b0a6a098d29ec7d82f64b607

Libraries/Fetch/fetch.js

@@ -390,8 +390,14 @@ var self = {};
       }
 
       xhr.onload = function() {
+        var status = (xhr.status === 1223) ? 204 : xhr.status
+        if (status < 100 || status > 599) {
+          reject(new TypeError('Network request failed'))
+          return
+        }
+
         var options = {
-          status: xhr.status,
+          status: status,
           statusText: xhr.statusText,
           headers: headers(xhr),
           url: responseURL()