Adds fastFail setting for flaky pages that aren't important

[hhff]

Description

There's a lot of people reporting Timeout errors, and if they're like me, they'd like to allow react-snap to crawl most of their site regardless of a couple weird page errors.

We have an app with a few hundred pages, and like, 2 of them are flaky. This allows those errors to be ignored, so we can build 99% of our pages.

💔Thank you for all your hard work on this lib!