stop behat on 500 errors with mink

I had some issues diagnosing 500 errors that were happening with Mink. The main issue being that a page would 500, but then the test would proceed and fail a later step. Unfortunately the HTML/screenshot that was captured does not capture the error, but instead captures the assertion which doesn’t help debug.

It’s possible to react after a step and check if the last Mink request was a 500 or not. This way, we fail on the 500, and not on the subsequent assertions.