feat: introduce puppeteer/Errors (#3056)

This patch adds a new require, `puppeteer/Errors`, that
holds all the Puppeteer-specific error classes.

Currently, the only custom error class we use is `TimeoutError`. We'll
expand in future with `CrashError` and some others.

Fixes #1694.

Errors.js

@@ -0,0 +1,28 @@
+/**
+ * Copyright 2018 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+let asyncawait = true;
+try {
+  new Function('async function test(){await 1}');
+} catch (error) {
+  asyncawait = false;
+}
+
+// If node does not support async await, use the compiled version.
+if (asyncawait)
+  module.exports = require('./lib/Errors');
+else
+  module.exports = require('./node6/lib/Errors');