Fix error msg for check `sample_fraction` param

[coveralls]

Pull Request Test Coverage Report for Build 78

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.368%

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Totals
Change from base Build 77:	0.0%
Covered Lines:	370
Relevant Lines:	380

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💛 - Coveralls

[thuijskens]

Thanks very much for the fix :+1: !