Add test for error message when no pages are found

https://github.com/cloudfour/lighthouse-parade/pull/30#issuecomment-700145996

import { scan } from './scan-task'
test('Displays useful error if no pages are found while crawling', () => {
  const result = await scan('https://nonexistent-website.com')
  expect(result.error).toMatch(/No pages were found for this site/)
})

We can get away with it for this test, but it’s hacky for tests to make live requests. Also very slow. Sooner than later we need to figure out a way to do this the right way. So maybe we take our time with that future issue when we get there.

[emersonthis]

I’ll create a new issue…

[emersonthis]

@calebeby how do you feel about taking this on?