I am still persuaded that something should be done in Moodle code to display a clear alert when a pre 2.1 incompatible question type is installed. This alert could also display the name of the offending question type and help to fix the problem more easily
This is why I created MDL-31099 tracker issue.
There are surely better ways to determine if a question type is compatible or not but as all pre 2.1 question types don't have a render.php nor a question.php file it would be quite easy to test if those 2 files exist or not.
This would no be bulletproof but it would catch a lot of problems.
If nothing is done, I am sure we will see other messages like this one in the future.