Use non-zero exit code on failure to start test runner

Thanks for bringing us Nightwatch, @beatfactor.  It's brilliant stuff.

Some of our CI builds were passing even when the call to CliRunner.init().runTests() failed, as they relied on exit codes to determine success/failure.  I understand that this is fairly standard for systems like Jenkins/Hudson.

Do you think it's reasonable to call process.exit(1) here?