storage.api.server: Add types to module and refactor tests

• Refactor the tests to simplify the reading (use pytest.raises' match behavior)
• Add types on impacted functions so the coverage shows as well

Not really for review. Preparatory work to fix configuration discrepancy which is the source of the docker-dev error.

Test Plan

tox

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Migrated from D4279 (view on Phabricator)

Build is green

Patch application report for D4279 (id=15134)

Rebasing onto 07c96743...

Current branch diff-target is up to date.

Changes applied before test

commit 934592a141bca2d494b9b02df5a802e4fc5bdc4e
Author: Antoine R. Dumont (@ardumont) <ardumont@softwareheritage.org>
Date:   Fri Oct 16 10:15:26 2020 +0200

    storage.api.server: Add types to module and refactor tests

See https://jenkins.softwareheritage.org/job/DCIDX/job/tests-on-diff/93/ for more details.

Merge request was merged

closed