[SPARK-4163][Core] Add a backward compatibility test for FetchFailed

/cc aarondav

Author: zsxwing <zsxwing@gmail.com>

Closes #3086 from zsxwing/SPARK-4163-back-comp and squashes the following commits:

21cb2a8 [zsxwing] Add a backward compatibility test for FetchFailed

core/src/test/scala/org/apache/spark/util/JsonProtocolSuite.scala

@@ -177,6 +177,17 @@ class JsonProtocolSuite extends FunSuite {
       deserializedBmRemoved)
   }
 
+  test("FetchFailed backwards compatibility") {
+    // FetchFailed in Spark 1.1.0 does not have an "Message" property.
+    val fetchFailed = FetchFailed(BlockManagerId("With or", "without you", 15), 17, 18, 19,
+      "ignored")
+    val oldEvent = JsonProtocol.taskEndReasonToJson(fetchFailed)
+      .removeField({ _._1 == "Message" })
+    val expectedFetchFailed = FetchFailed(BlockManagerId("With or", "without you", 15), 17, 18, 19,
+      "Unknown reason")
+    assert(expectedFetchFailed === JsonProtocol.taskEndReasonFromJson(oldEvent))
+  }
+
   test("SparkListenerApplicationStart backwards compatibility") {
     // SparkListenerApplicationStart in Spark 1.0.0 do not have an "appId" property.
     val applicationStart = SparkListenerApplicationStart("test", None, 1L, "user")