vault: test_server: Fix exception structure

swh.core [2] changed the remote exception serialization handling.

So the current build [1] is broken. It needs adaptation to retrieve the exception information at its first level. This adapts accordingly the failing tests.

• [1] https://jenkins.softwareheritage.org/job/DVAU/job/tests/881/

• [2] Related to swh-core!187 (closed) (released in swh.core 0.9.0)

Test Plan

tox

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Migrated from D4553 (view on Phabricator)