Low percentage gives zero masks

[egpbos]

In the distance method, if you don't have a lot of masks and use a low distance filtering percentage, you can end up with zero masks, which will crash the code. We should build in a check for this that fails gracefully.

The problem can be detected early on in explain_image_distance. If you for instance have 10 masks and your filter percentage is anything below 10, you won't get any mask.