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.

docs/api.md

@@ -159,6 +159,7 @@
 - [class: Request](#class-request)
   * [request.abort()](#requestabort)
   * [request.continue([overrides])](#requestcontinueoverrides)
+  * [request.failure()](#requestfailure)
   * [request.headers](#requestheaders)
   * [request.method](#requestmethod)
   * [request.postData](#requestpostdata)
@@ -1817,6 +1818,21 @@ Exception is immediately thrown if the request interception is not enabled.
 Continues request with optional request overrides. To use this, request interception should be enabled with `page.setRequestInterceptionEnabled`.
 Exception is immediately thrown if the request interception is not enabled.
 
+#### request.failure()
+- returns: <[Object]> Object describing request failure, if any
+  - `errorText` <[string]> Human-readable error message, e.g. `'net::ERR_FAILED'`.
+
+The method returns null unless this request was failed, as reported by
+`requestfailed` event.
+
+Example of logging all failed requests:
+
+```js
+page.on('requestfailed', request => {
+  console.log(request.url + ' ' + request.failure().errorText);
+});
+```
+
 #### request.headers
 - <[Object]> An object with HTTP headers associated with the request. All header names are lower-case.
 

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
    */

test/test.js

@@ -2125,6 +2125,7 @@ describe('Page', function() {
       expect(failedRequests[0].url).toContain('one-style.css');
       expect(failedRequests[0].response()).toBe(null);
       expect(failedRequests[0].resourceType).toBe('stylesheet');
+      expect(failedRequests[0].failure().errorText).toBe('net::ERR_FAILED');
     }));
     it('Page.Events.RequestFinished', SX(async function() {
       const requests = [];