auth.pytest_plugin: Adjust error type according to reality

Prior to this commit, this raised a basic error message instead of a json dict bytes encoded [1]

This commit fixes it.

The reality of tests trying to fix the deposit authentication handling which is awkward for now.

Related to swh-deposit#2858 (closed) Related to swh-deposit!226 (closed)

• [1] Related to swh/meta$978

Test Plan

tox

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Migrated from D5304 (view on Phabricator)