I'm just getting up to speed on WPF, MVVM and Prism 6. I understand how to use IDataErrorInfo but I don't know why or how you would use IDataErrorInfo.Error. In every example I've seen, it's get method returns null or a not implemented exception. It has to be in the class or you get a compile error.
What is it used for?
From my experience, I can tell you that this field is very useful - it's used basically to provide a tooltip about the error to the user.
for an example, take a look here