Show better warning message when negative value is supplied for explanatory variable

[myui]

Show better warning message when negative value is supplied for explanatory variable

Caused by: org.apache.hadoop.hive.ql.metadata.HiveException: java.lang.NegativeArraySizeException
        at hivemall.smile.classification.RandomForestClassifierUDTF.train(RandomForestClassifierUDTF.java:278)
        at hivemall.smile.classification.RandomForestClassifierUDTF.close(RandomForestClassifierUDTF.java:214)
        at org.apache.hadoop.hive.ql.exec.UDTFOperator.closeOp(UDTFOperator.java:145)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:616)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:630)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:630)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:630)
        at org.apache.hadoop.hive.ql.exec.tez.MapRecordProcessor.close(MapRecordProcessor.java:311)
        ... 14 more
Caused by: java.lang.NegativeArraySizeException
        at hivemall.smile.classification.DecisionTree$TrainNode.findBestSplit(DecisionTree.java:462)
        at hivemall.smile.classification.DecisionTree$TrainNode.findBestSplit(DecisionTree.java:428)
        at hivemall.smile.classification.DecisionTree$TrainNode.split(DecisionTree.java:617)
        at hivemall.smile.classification.DecisionTree$TrainNode.split(DecisionTree.java:612)
        at hivemall.smile.classification.DecisionTree$TrainNode.split(DecisionTree.java:612)
        at hivemall.smile.classification.DecisionTree.<init>(DecisionTree.java:764)
        at hivemall.smile.classification.RandomForestClassifierUDTF$TrainingTask.call(RandomForestClassifierUDTF.java:385)
        at hivemall.smile.classification.RandomForestClassifierUDTF$TrainingTask.call(RandomForestClassifierUDTF.java:314)
        at hivemall.smile.utils.SmileTaskExecutor.run(SmileTaskExecutor.java:75)
        at hivemall.smile.classification.RandomForestClassifierUDTF.train(RandomForestClassifierUDTF.java:276)