[SPARK-19886] Fix reportDataLoss if statement in SS KafkaSource

## What changes were proposed in this pull request?

Fix the `throw new IllegalStateException` if statement part.

## How is this patch tested

Regression test

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #17228 from brkyvz/kafka-cause-fix.

Author: brkyvz

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/CachedKafkaConsumer.scala

@@ -273,19 +273,7 @@ private[kafka010] case class CachedKafkaConsumer private(
       message: String,
       cause: Throwable = null): Unit = {
     val finalMessage = s"$message ${additionalMessage(failOnDataLoss)}"
-    if (failOnDataLoss) {
-      if (cause != null) {
-        throw new IllegalStateException(finalMessage)
-      } else {
-        throw new IllegalStateException(finalMessage, cause)
-      }
-    } else {
-      if (cause != null) {
-        logWarning(finalMessage)
-      } else {
-        logWarning(finalMessage, cause)
-      }
-    }
+    reportDataLoss0(failOnDataLoss, finalMessage, cause)
   }
 
   private def close(): Unit = consumer.close()
@@ -398,4 +386,23 @@ private[kafka010] object CachedKafkaConsumer extends Logging {
       consumer
     }
   }
+
+  private def reportDataLoss0(
+      failOnDataLoss: Boolean,
+      finalMessage: String,
+      cause: Throwable = null): Unit = {
+    if (failOnDataLoss) {
+      if (cause != null) {
+        throw new IllegalStateException(finalMessage, cause)
+      } else {
+        throw new IllegalStateException(finalMessage)
+      }
+    } else {
+      if (cause != null) {
+        logWarning(finalMessage, cause)
+      } else {
+        logWarning(finalMessage)
+      }
+    }
+  }
 }

external/kafka-0-10-sql/src/test/scala/org/apache/spark/sql/kafka010/CachedKafkaConsumerSuite.scala

@@ -0,0 +1,34 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.kafka010
+
+import org.scalatest.PrivateMethodTester
+
+import org.apache.spark.sql.test.SharedSQLContext
+
+class CachedKafkaConsumerSuite extends SharedSQLContext with PrivateMethodTester {
+
+  test("SPARK-19886: Report error cause correctly in reportDataLoss") {
+    val cause = new Exception("D'oh!")
+    val reportDataLoss = PrivateMethod[Unit]('reportDataLoss0)
+    val e = intercept[IllegalStateException] {
+      CachedKafkaConsumer.invokePrivate(reportDataLoss(true, "message", cause))
+    }
+    assert(e.getCause === cause)
+  }
+}