Ability to fail test when there is no reference image

[mgibas]

I would like to fail a test when there is no reference image. My use case looks like that:

• run tests on local machine (generates reference images)
• inspect local images
• commit, push
• run tests on CI (generates images for different browsers, OS)
• download images and inspect
• commit and push downloaded images as reference

there is possibility though that someone will forget some files or these steps so It would be nice to have an option to fail test run that doesn't have reference images and they need to be copied over from taken to referene manually to make next run green.

[zinserjan]

Letting the tests fail is out of responsibility of this service. But we could provide the required information for this into the result reports, so you can assert it by yourself.

Implementation should be simple when #78 is merged.

[mgibas]

That seems like a fair solution :+1: