[MRG] Raise error when subsample does not contain samples of one class

[thuijskens]

…in case of classification

[coveralls]

Pull Request Test Coverage Report for Build 51

• 7 of 11 (63.64%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 87.24%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
stability_selection/stability_selection.py	7	11	63.64%
<!--	Total:	7	11	63.64%	-->

Totals
Change from base Build 49:	-0.8%
Covered Lines:	294
Relevant Lines:	337

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💛 - Coveralls