Resolve pending callbacks when connection closes

This patch:
- resolves pending callbacks in connection once it gets closed
- fixes one of Page.screenshot() tests to wait for all the screenshots
- starts handling ECONNRESET error in test server to avoid throwing with no
  good reason

lib/Connection.js

@@ -76,6 +76,8 @@ class Connection extends EventEmitter {
   _onClose() {
     this._ws.removeAllListeners();
     this._ws.close();
+    for (let callback of this._callbacks.values())
+      callback.reject(new Error(`Protocol error (${callback.method}): Target closed.`));
   }
 
   /**

test/server/SimpleServer.js

@@ -79,6 +79,12 @@ class SimpleServer {
 
   _onSocket(socket) {
     this._sockets.add(socket);
+    // ECONNRESET is a legit error given
+    // that tab closing simply kills process.
+    socket.on('error', error => {
+      if (error.code !== 'ECONNRESET')
+        throw error;
+    });
     socket.once('close', () => this._sockets.delete(socket));
   }
 

test/test.js

@@ -628,8 +628,8 @@ describe('Puppeteer', function() {
           }
         }));
       }
-      let screenshot = await promises[1];
-      expect(screenshot).toBeGolden('screenshot-parallel-calls.png');
+      let screenshots = await Promise.all(promises);
+      expect(screenshots[1]).toBeGolden('screenshot-parallel-calls.png');
     }));
     it('should take fullPage screenshots', SX(async function() {
       await page.setViewport({width: 500, height: 500});