Node.js v6 was end-of-life'd in April, 2019, with AWS Lambda prohibiting updaets to the Node.js v6 runtime since June 30, 2019.
This makes it quite safe for us to remove the Node 6 support from the repository.
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.
