lib: migrate _http_outgoing.js's remaining errors

A couple of lib/_http_outgoing.js's errors were still in the
"old style": `throw new Error(<some message here>)`.

This commit migrates those 2 old style errors to the "new style":
internal/errors.js's error-system.

In the future, changes to these errors' messages won't break
semver-major status. With the old style, changes to these errors'
messages broke semver-major status. It was inconvenient.

Refs: https://github.com/nodejs/node/issues/17709
PR-URL: https://github.com/nodejs/node/pull/17837
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_http_outgoing.js

@@ -629,7 +629,7 @@ OutgoingMessage.prototype.write = function write(chunk, encoding, callback) {
 
 function write_(msg, chunk, encoding, callback, fromEnd) {
   if (msg.finished) {
-    var err = new Error('write after end');
+    const err = new errors.Error('ERR_STREAM_WRITE_AFTER_END');
     nextTick(msg.socket && msg.socket[async_id_symbol],
              writeAfterEndNT.bind(msg),
              err,
@@ -880,7 +880,7 @@ OutgoingMessage.prototype.flush = internalUtil.deprecate(function() {
 
 OutgoingMessage.prototype.pipe = function pipe() {
   // OutgoingMessage should be write-only. Piping from it is disabled.
-  this.emit('error', new Error('Cannot pipe, not readable'));
+  this.emit('error', new errors.Error('ERR_STREAM_CANNOT_PIPE'));
 };
 
 module.exports = {

test/parallel/test-http-res-write-after-end.js

@@ -25,8 +25,9 @@ const assert = require('assert');
 const http = require('http');
 
 const server = http.Server(common.mustCall(function(req, res) {
-  res.on('error', common.mustCall(function onResError(err) {
-    assert.strictEqual(err.message, 'write after end');
+  res.on('error', common.expectsError({
+    code: 'ERR_STREAM_WRITE_AFTER_END',
+    type: Error
   }));
 
   res.write('This should write.');

test/parallel/test-http-server-write-after-end.js

@@ -2,7 +2,6 @@
 
 const common = require('../common');
 const http = require('http');
-const assert = require('assert');
 
 // Fix for https://github.com/nodejs/node/issues/14368
 
@@ -10,7 +9,10 @@ const server = http.createServer(handle);
 
 function handle(req, res) {
   res.on('error', common.mustCall((err) => {
-    assert.strictEqual(err.message, 'write after end');
+    common.expectsError({
+      code: 'ERR_STREAM_WRITE_AFTER_END',
+      type: Error
+    })(err);
     server.close();
   }));
 

test/parallel/test-outgoing-message-pipe.js

@@ -1,15 +1,14 @@
 'use strict';
-const assert = require('assert');
 const common = require('../common');
 const OutgoingMessage = require('_http_outgoing').OutgoingMessage;
 
 // Verify that an error is thrown upon a call to `OutgoingMessage.pipe`.
 
 const outgoingMessage = new OutgoingMessage();
-assert.throws(
-  common.mustCall(() => { outgoingMessage.pipe(outgoingMessage); }),
-  (err) => {
-    return ((err instanceof Error) && /Cannot pipe, not readable/.test(err));
-  },
-  'OutgoingMessage.pipe should throw an error'
+common.expectsError(
+  () => { outgoingMessage.pipe(outgoingMessage); },
+  {
+    code: 'ERR_STREAM_CANNOT_PIPE',
+    type: Error
+  }
 );

test/parallel/test-pipe-outgoing-message-data-emitted-after-ended.js

@@ -1,7 +1,6 @@
 'use strict';
 const common = require('../common');
 const http = require('http');
-const assert = require('assert');
 const stream = require('stream');
 
 // Verify that when piping a stream to an `OutgoingMessage` (or a type that
@@ -17,8 +16,9 @@ const server = http.createServer(common.mustCall(function(req, res) {
   process.nextTick(common.mustCall(() => {
     res.end();
     myStream.emit('data', 'some data');
-    res.on('error', common.mustCall(function(err) {
-      assert.strictEqual(err.message, 'write after end');
+    res.on('error', common.expectsError({
+      code: 'ERR_STREAM_WRITE_AFTER_END',
+      type: Error
     }));
 
     process.nextTick(common.mustCall(() => server.close()));