feat(Request): introduce Request.failure() method (#1065)

Similarly to the `request.response()` method, this patch adds
`request.failure()` method that returns error details for the failed
requests.

Fixes #901.

lib/NetworkManager.js

@@ -259,6 +259,7 @@ class NetworkManager extends EventEmitter {
     // @see https://crbug.com/750469
     if (!request)
       return;
+    request._failureText = event.errorText;
     request._completePromiseFulfill.call(null);
     this._requestIdToRequest.delete(request._requestId);
     this._interceptionIdToRequest.delete(request._interceptionId);
@@ -284,6 +285,7 @@ class Request {
     this._allowInterception = allowInterception;
     this._interceptionHandled = false;
     this._response = null;
+    this._failureText = null;
     this._completePromise = new Promise(fulfill => {
       this._completePromiseFulfill = fulfill;
     });
@@ -304,6 +306,17 @@ class Request {
     return this._response;
   }
 
+  /**
+   * @return {?{errorText: string}}
+   */
+  failure() {
+    if (!this._failureText)
+      return null;
+    return {
+      errorText: this._failureText
+    };
+  }
+
   /**
    * @param {!Object=} overrides
    */