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