fix(cli): do not error unneccesarily on session stop

[Panaetius]

Fixes #3589

[coveralls]

Pull Request Test Coverage Report for Build 6822655139

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• 0 of 8 (0.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 82.825%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/core/session/session.py	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
renku/core/migration/m_0003__1_jsonld.py	1	87.68%
renku/ui/cli/init.py	1	96.9%
renku/ui/cli/service.py	1	28.57%

Totals
Change from base Build 6822651972:	-0.02%
Covered Lines:	25453
Relevant Lines:	30731

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💛 - Coveralls