Cancel Algorithm Runs when Started with Non-Existing Examples

[JensLehmann]

┆Issue is synchronized with this Trello card by Unito

[patrickwestphal]

Learning problems with example checks:

• PosOnlyLP
• ClassExpressionLearningProblem
• PosNegLP
• PosNegLPStrict
• PosNegLPStandard
• PosNegUndLP
• FuzzyPosNegLP
• FuzzyPosNegLPStandard
• ClassLearningProblem
• ClassAsInstanceLearningProblem