Provide more information in validator error messages

[rmkubik]

We should be able to provide more info in our validator error messages in two cases:

1. We cannot find the file we're looking for, meaning the player likely has a typo in their file name
2. We cannot find their Python executable any more, this could happen if the player updates their python installation in a way that changes the path. Paths to installations can often include version numbers like this.