more fault tolerant for shell setting

[connectdotz]

Make "jest.shell" setting init more fault-tolerant.

Resolve #1099

[coveralls]

Pull Request Test Coverage Report for Build 7515296503

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 97.945%

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Totals
Change from base Build 6764758206:	0.002%
Covered Lines:	3912
Relevant Lines:	3920

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💛 - Coveralls