fix(service): return proper errors on migrations check

[Panaetius]

closes #2124

[coveralls]

Pull Request Test Coverage Report for Build 5134481146

• 197 of 222 (88.74%) changed or added relevant lines in 24 files are covered.
• 9 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.06%) to 85.371%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/command/checks/githooks.py	3	4	75.0%
renku/command/checks/project.py	3	4	75.0%
renku/command/checks/storage.py	2	3	66.67%
renku/command/checks/workflow.py	7	8	87.5%
renku/command/checks/validate_shacl.py	2	4	50.0%
renku/ui/cli/migrate.py	4	6	66.67%
renku/ui/service/controllers/cache_migrations_check.py	16	18	88.89%
renku/ui/service/serializers/cache.py	25	27	92.59%
renku/command/checks/activities.py	2	5	40.0%
renku/command/checks/datasets.py	8	11	72.73%

Files with Coverage Reduction	New Missed Lines	%
renku/core/dataset/providers/dataverse.py	1	64.76%
renku/core/workflow/activity.py	1	93.61%
renku/ui/cli/init.py	1	96.9%
renku/ui/cli/service.py	1	68.91%
renku/domain_model/template.py	2	90.68%
renku/ui/service/gateways/gitlab_api_provider.py	3	88.64%

Totals
Change from base Build 5134475966:	0.06%
Covered Lines:	25824
Relevant Lines:	30249

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💛 - Coveralls

[m-alisafaee]

LGTM, please see my comments.

[m-alisafaee]

Thanks Ralf! It's mainly about docstring updates. Should we use a named-tuple for the return value of doctor checks? It's getting more complicated.