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:

• [x] PosOnlyLP
• [x] ClassExpressionLearningProblem
• [x] PosNegLP
• [x] PosNegLPStrict
• [x] PosNegLPStandard
• [x] PosNegUndLP
• [x] FuzzyPosNegLP
• [x] FuzzyPosNegLPStandard
• [x] ClassLearningProblem
• [x] ClassAsInstanceLearningProblem