tests: Fix a test 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-mercurial.log
Staging instance, all changes can be removed at any time
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-mercurial.log