Add support for ontimeout and onerror handler when using XMLHttpRequest for Android and iOS

Summary:Currently React-Native does not have `ontimeout` and `onerror` handlers for [XMLHttpRequest](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest). This is an extension to [No timeout on XMLHttpRequest](https://github.com/facebook/react-native/issues/4648).

With addition to two handlers, both Android and iOS can now handle `ontimeout` if request times out and `onerror` when there is general network error.

**Test plan**

Code has been tested on both Android and iOS with [Charles](https://www.charlesproxy.com/) by setting a breakpoint on the request which fires `ontimeout` when the request waits beyond `timeout` time and `onerror` when there is network error.

**Usage**

JavaScript -

```
var request = new XMLHttpRequest();

function onLoad() {
    console.log(request.status);
};

function onTimeout() {
    console.log('Timeout');
};

function onError() {
    console.log('General network error');
};

request.onload = onLoad;
request.ontimeout = onTimeout;
request.onerr
Closes https://github.com/facebook/react-native/pull/6841

Differential Revision: D3178859

Pulled By: lexs

fb-gh-sync-id: 30674570653e92ab5f7e74bd925dd5640fc862b6
fbshipit-source-id: 30674570653e92ab5f7e74bd925dd5640fc862b6

Libraries/Network/RCTNetworking.m

@@ -385,6 +385,7 @@ RCT_EXPORT_MODULE()
       }
       NSArray *responseJSON = @[task.requestID,
                                 RCTNullIfNil(error.localizedDescription),
+                                error.code == kCFURLErrorTimedOut ? @YES : @NO
                                 ];
 
       [_bridge.eventDispatcher sendDeviceEventWithName:@"didCompleteNetworkResponse"