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.
Today, we have tests split into multiple files, with files pulling
tests from some other files.
This patch starts explicitly gathering all tests from the same
`test.js` file.
Drive-By: move one test from `browser.spec.js` into `puppeteer.spec.js`
since it starts browser itself.
