Testing with Nightmare V2 based on Electron , Mocha and Chai. Command "mocha" doesn't catch the errors

I use Nightmare with Mocha and Chai for testing my website. Initially Nightmare was based on phantomjs but with the new release of the version2 it is used electron. Even if I have forced 500 error the test it's still passed. I'm now running my test using "mocha" command in my terminal. Is there another command for running test? Or maybe someone had the same issue as me and knows how to fix this.