[SPARK-3089] Fix meaningless error message in ConnectionManager

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2000 from sarutak/SPARK-3089 and squashes the following commits:

02dfdea [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-3089
e759ce7 [Kousuke Saruta] Improved error message when closing SendingConnection

core/src/main/scala/org/apache/spark/network/ConnectionManager.scala

@@ -467,7 +467,7 @@ private[spark] class ConnectionManager(
 
           val sendingConnectionOpt = connectionsById.get(remoteConnectionManagerId)
           if (!sendingConnectionOpt.isDefined) {
-            logError("Corresponding SendingConnectionManagerId not found")
+            logError(s"Corresponding SendingConnection to ${remoteConnectionManagerId} not found")
             return
           }