Changing the flag to failOnSevere was to mirror protractor-console-plugin more closely

I want to add failOnWarning too, but first I want to figure out why the level here is called severe instead of error

PR isn't ready actually!!! I need to make the addFailure actually work (there's a test in there already for asserting that it exists, which currently it does not)

Figured it out 🙂 although now I'm wishing everything was written with type annotations

Thanks for the great contribution @lynncyrin! Would you mind also updating the package.json version (probably 3.1.0 since this is a new feature) and an entry to the CHANGELOG?

So it turns out that I ended up not needing this, and moved on to other things! Someone else is free to grab taking this across the finish line