tests: Fix numerous tests after recent changes in loader-core

BaseLoader.load now returns a dict with an extra error field when a loading fails.

See https://jenkins.softwareheritage.org/job/DENV/job/tests/758/artifact/swh-loader-git.log

Jenkins job DLDG/gitlab-builds #197 succeeded .
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approved this merge request

added 2 commits

• 48d0acfa - 1 commit from branch swh/devel:master
• 0dd744df - tests: Fix numerous tests after recent changes in loader-core

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Jenkins job DLDG/gitlab-builds #200 succeeded in 1 min 50 sec.
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merged manually