[SPARK-19398] Change one misleading log in TaskSetManager.

## What changes were proposed in this pull request?

Log below is misleading:

```
if (successful(index)) {
  logInfo(
    s"Task ${info.id} in stage ${taskSet.id} (TID $tid) failed, " +
    "but another instance of the task has already succeeded, " +
    "so not re-queuing the task to be re-executed.")
}
```

If fetch failed, the task is marked as successful in `TaskSetManager:: handleFailedTask`. Then log above will be printed. The `successful` just means task will not be scheduled any longer, not a real success.

## How was this patch tested?
Existing unit tests can cover this.

Author: jinxing <jinxing@meituan.com>

Closes #16738 from jinxing64/SPARK-19398.

core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala

@@ -70,6 +70,10 @@ private[spark] class TaskSetManager(
   val tasks = taskSet.tasks
   val numTasks = tasks.length
   val copiesRunning = new Array[Int](numTasks)
+
+  // For each task, tracks whether a copy of the task has succeeded. A task will also be
+  // marked as "succeeded" if it failed with a fetch failure, in which case it should not
+  // be re-run because the missing map data needs to be regenerated first.
   val successful = new Array[Boolean](numTasks)
   private val numFailures = new Array[Int](numTasks)
 
@@ -797,10 +801,10 @@ private[spark] class TaskSetManager(
     sched.dagScheduler.taskEnded(tasks(index), reason, null, accumUpdates, info)
 
     if (successful(index)) {
-      logInfo(
-        s"Task ${info.id} in stage ${taskSet.id} (TID $tid) failed, " +
-        "but another instance of the task has already succeeded, " +
-        "so not re-queuing the task to be re-executed.")
+      logInfo(s"Task ${info.id} in stage ${taskSet.id} (TID $tid) failed, but the task will not" +
+        s" be re-executed (either because the task failed with a shuffle data fetch failure," +
+        s" so the previous stage needs to be re-run, or because a different copy of the task" +
+        s" has already succeeded).")
     } else {
       addPendingTask(index)
     }