[MINOR][CORE] When failed to canceling the job group, add a warning log

### What changes were proposed in this pull request?
The pr aims to add add a warning log when failed to canceling the job group.

### Why are the changes needed?
In production, we found that when there are many jobs, if we specify `a just-submitted job group` and `cancel` it using the API `SparkContext#cancelJobGroup(groupId: String)`, the cancellation will fail (`silently`) because the job has not yet entered `activeJobs`, and there is `no warning` prompt.
(PS: Due to historical reasons, we are not currently using the newer API `SparkContext#cancelJobGroupAndFutureJobs(groupId: String)`)

I propose to add `a warning log` here to help troubleshoot `issues`.

### Does this PR introduce _any_ user-facing change?
Yes, only for log.

### How was this patch tested?
N/A

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#45722 from panbingkun/handleJobGroupCancelled_add_log.

Authored-by: panbingkun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

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

@@ -1220,6 +1220,9 @@ private[spark] class DAGScheduler(
         _.getProperty(SparkContext.SPARK_JOB_GROUP_ID) == groupId
       }
     }
+    if (activeInGroup.isEmpty && !cancelFutureJobs) {
+      logWarning(s"Failed to cancel job group $groupId. Cannot find active jobs for it.")
+    }
     val jobIds = activeInGroup.map(_.jobId)
     jobIds.foreach(handleJobCancellation(_,
         Option("part of cancelled job group %s".format(groupId))))