Add configuration to fail test if baseline image doesn't exists

[patrickclerc]

Hello

Here is a feature/configuration proposal : currently I push my baseline images in my git repository and my CI is running automatically. The tests are passing in the case I forget to push the baseline images, but the test is not comparing images, and the pipeline gives a false success. It would be great to have an option to fail the tests if no baseline images exists.

Thank you for your very useful tool !