This is a tiny package which shows failure of rostest to show errors in the test scripts.
This seems to only be a problem when the instantiation of the unittest class fails. Errors in the actual code of test functions will display normally with a traceback.
Put the package into your catkin workspace, and build it
catkin build rostest_example
The test_error.py test file has a deliberate error in it which will cause the test to return an error.
Run the test with
catkin test rostest_example
The test result will be an error, but the cause of the error will not be displayed.
If the test is run with
python3 test/test_error.py
With this, you will see the cause of the error, although without a traceback.