[Backport 2.x] Fix to handle the exception for deletion of non-existing autofollow replication rule

src/main/kotlin/org/opensearch/replication/action/autofollow/TransportAutoFollowClusterManagerNodeAction.kt

@@ -123,6 +123,7 @@ class TransportAutoFollowClusterManagerNodeAction @Inject constructor(transportS
         } catch(e: ResourceNotFoundException) {
             // Log warn as the task is already removed
             log.warn("Task already stopped for '$clusterAlias:$patternName'", e)
+            throw OpenSearchException("Autofollow replication rule $clusterAlias:$patternName does not exist")
         } catch (e: Exception) {
            log.error("Failed to stop auto follow task for cluster '$clusterAlias:$patternName'", e)
             throw OpenSearchException(AUTOFOLLOW_EXCEPTION_GENERIC_STRING)

src/test/kotlin/org/opensearch/replication/integ/rest/UpdateAutoFollowPatternIT.kt

@@ -295,6 +295,22 @@ class UpdateAutoFollowPatternIT: MultiClusterRestTestCase() {
             .hasMessageContaining(errorMsg)
     }
 
+    fun `test deletion of auto follow pattern`() {
+        val followerClient = getClientForCluster(FOLLOWER)
+        createConnectionBetweenClusters(FOLLOWER, LEADER, connectionAlias)
+        followerClient.updateAutoFollowPattern(connectionAlias, indexPatternName, indexPattern)
+        //Delete a replication rule which does not exist
+        Assertions.assertThatThrownBy {
+            followerClient.deleteAutoFollowPattern(connectionAlias, "dummy_conn")
+        }.isInstanceOf(ResponseException::class.java)
+                .hasMessageContaining("does not exist")
+        //Delete a replication rule which exists
+        Assertions.assertThatCode {
+            followerClient.deleteAutoFollowPattern(connectionAlias, indexPatternName)
+        }.doesNotThrowAnyException()
+
+    }
+
     fun `test removing autofollow pattern stop autofollow task`() {
         val followerClient = getClientForCluster(FOLLOWER)
         val leaderClient = getClientForCluster(LEADER)