This patch introduces API to manage frame navigations.
As a drive-by, the `response.frame()` method is added as a shortcut
for `response.request().frame()`.
Fixes#2918.
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.
