MINOR: Log when remote fetch execution is rejected (#14350)

Log at warning level when remote fetch execution is rejected. This can be potentially useful to troubleshoot UNKNOWN_SERVER_ERROR responses for remote fetch requests.

Reviewers: Luke Chen <showuon@gmail.com>, Christo Lolov <lolovc@amazon.com>

core/src/main/scala/kafka/server/ReplicaManager.scala

@@ -1263,6 +1263,7 @@ class ReplicaManager(val config: KafkaConfig,
     } catch {
       case e: RejectedExecutionException =>
         // Return the error if any in scheduling the remote fetch task
+        warn("Unable to fetch data from remote storage", e)
         return Some(createLogReadResult(e))
     }