Scala test fw

First attempt to use the inhouse Scala testing framework with one test for checking the picture overlay functionality. Keep in mind that I am a beginner Scala dev so code will likely look very "Javaish"
Try to avoid using `Exception`s to return what is essentially information in Scala. They're not nice, as they don't show up in type signatures, so people have to read your method to understand what it's actually doing.

Given that this method can only return either `true` or throw an `Exception`, why not just return an `Option[String]`?

Then it's either `Some(errorMessage)` or `None` if everything is OK.