Fixes #16

This fix adds user-friendly error for the case when field names in data class and respective field names in dataset are not consistent.

Signed-off-by: Pasha Finkelshteyn <asm0dey@jetbrains.com>

Author: asm0dey

core/src/main/scala/org/apache/spark/sql/KotlinReflection.scala

@@ -655,6 +655,8 @@ object KotlinReflection extends KotlinReflection {
             val fields = properties.map { prop =>
               val fieldName = prop.getName
               val maybeField = dataType.dt.fields.find(it => it.name == fieldName)
+              if (maybeField.isEmpty)
+                throw new IllegalArgumentException(s"Field $fieldName is not found among available fields, which are: ${dataType.dt.fields.map(_.name).mkString(", ")}")
               val propClass = maybeField.map(it => it.dataType.asInstanceOf[DataTypeWithClass].cls).get
               val propDt = maybeField.map(it => it.dataType.asInstanceOf[DataTypeWithClass]).get