failure is assertion failure.
error is any other error.

Source: https://docs.python.org/3/library/unittest.html#organizing-test-code
-> "If the test fails, an exception will be raised ... identify the test
case as a failure. Any other exceptions will be treated as errors"

While searching ERP5 code for failure_count to clarify its meaning, I've
also discovered expected_failures and unexpected_successes (*) so they
are also added to the table.

(*) see e.g. https://lab.nexedi.com/nexedi/erp5/blob/1e31c091/product/ERP5Type/tests/ERP5TypeTestSuite.py#L6-8