add getter as public api for ErrorUtils._globalHandler

Summary:As discussed here #1194 . This add an getter the default handler, so that custom handler can be added (monkey patch?) without overwriting the default handler
Closes https://github.com/facebook/react-native/pull/5575

Differential Revision: D2948994

fb-gh-sync-id: 2b6d1619cfe68f78b326c6d232b9bf57c489c45d
shipit-source-id: 2b6d1619cfe68f78b326c6d232b9bf57c489c45d

packager/react-packager/src/Resolver/polyfills/error-guard.js

@@ -20,6 +20,9 @@ var ErrorUtils = {
   setGlobalHandler: function(fun) {
     ErrorUtils._globalHandler = fun;
   },
+  getGlobalHandler: function() {
+    return ErrorUtils._globalHandler;
+  },
   reportError: function(error) {
     ErrorUtils._globalHandler && ErrorUtils._globalHandler(error);
   },