swh.core (0.9.0) changed the remote exception serialization handling.
So the current build [1] is broken as it needs adaptation to retrieve the
exception information at its first level.
This adapts accordingly the exception handling in those failing tests.
[1] https://jenkins.softwareheritage.org/job/DVAU/job/tests/881/
Related to D4515 (released in swh.core 0.9.0)