Determine failed navigation based on Frame.unreachedURL (#365)

This patch starts using frame.unreachedURL property from
frameNavigated event to determine if the page navigation
was successful.

lib/FrameManager.js

@@ -130,6 +130,14 @@ class FrameManager extends EventEmitter {
     this._frames.delete(frame._id);
     this.emit(FrameManager.Events.FrameDetached, frame);
   }
+
+  /**
+   * @param {!Frame} frame
+   * @return {boolean}
+   */
+  isMainFrameLoadingFailed() {
+    return !!this._mainFrame._loadingFailed;
+  }
 }
 
 /** @enum {string} */
@@ -337,6 +345,7 @@ class Frame {
   _navigated(framePayload) {
     this._name = framePayload.name;
     this._url = framePayload.url;
+    this._loadingFailed = !!framePayload.unreachableUrl;
   }
 
   _detach() {