Handling errors

I have an idea for handling errors.

Normally you would deny faulty data to be inserted into the database before it happens.
Currently, we have a start table, that has valid data. We could change this method to insert one row after another and let the students create checks and catch the error message somehow.
I am unsure how obvious these error test cases would be for those who read our test.sql file.

As an alternative, we could ask them to clean erroneous data from the table and check if it is still there.

Do you have other ideas?