[SPARK-11343][ML] Documentation of float and double prediction/label columns in RegressionEvaluator

felixcheung , mengxr

Just added a message to require()

Author: Dominik Dahlem <dominik.dahlem@gmail.combination>

Closes #9598 from dahlem/ddahlem_regression_evaluator_double_predictions_message_04112015.

mllib/src/main/scala/org/apache/spark/ml/evaluation/RegressionEvaluator.scala

@@ -73,10 +73,15 @@ final class RegressionEvaluator @Since("1.4.0") (@Since("1.4.0") override val ui
   @Since("1.4.0")
   override def evaluate(dataset: DataFrame): Double = {
     val schema = dataset.schema
+    val predictionColName = $(predictionCol)
     val predictionType = schema($(predictionCol)).dataType
-    require(predictionType == FloatType || predictionType == DoubleType)
+    require(predictionType == FloatType || predictionType == DoubleType,
+      s"Prediction column $predictionColName must be of type float or double, " +
+        s" but not $predictionType")
+    val labelColName = $(labelCol)
     val labelType = schema($(labelCol)).dataType
-    require(labelType == FloatType || labelType == DoubleType)
+    require(labelType == FloatType || labelType == DoubleType,
+      s"Label column $labelColName must be of type float or double, but not $labelType")
 
     val predictionAndLabels = dataset
       .select(col($(predictionCol)).cast(DoubleType), col($(labelCol)).cast(DoubleType))