INT-4075: Fix NPE in AbstractStompSessionManager

JIRA: https://jira.spring.io/browse/INT-4075

Fixes GH-1854 (https://github.com/spring-projects/spring-integration/issues/1854)

The `catch` block just swallows an Exception with the logger and leaves `stompSessionListenableFuture` as `null`.
The next `stompSessionListenableFuture` usage over two code lines bellow ends up with `NPE`.

* Add reconnect logic into that `catch` block and just return from there, since we don't have anything to go ahead.
* Add `if (e != null) {` around `log.error` in the `scheduleReconnect`
* Add `if...else` around `this.stompClient.getTaskScheduler()` and `log.info()` to notify end-user that there won't be reconnection if there is no `taskScheduler`
* Add `StompSessionManagerTests` mock tests to verify solution

**Cherry-pick to 4.2.x**

spring-integration-stomp/src/test/java/org/springframework/integration/stomp/StompSessionManagerTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.stomp;
+
+import static org.junit.Assert.assertNotNull;
+import static org.mockito.Mockito.mock;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import org.junit.Test;
+
+import org.springframework.messaging.simp.stomp.StompClientSupport;
+import org.springframework.messaging.simp.stomp.StompHeaders;
+import org.springframework.messaging.simp.stomp.StompSession;
+import org.springframework.messaging.simp.stomp.StompSessionHandler;
+import org.springframework.messaging.simp.stomp.StompSessionHandlerAdapter;
+import org.springframework.scheduling.concurrent.ConcurrentTaskScheduler;
+import org.springframework.util.concurrent.ListenableFuture;
+import org.springframework.util.concurrent.SettableListenableFuture;
+
+/**
+ * @author Artem Bilan
+ * @since 4.2.9
+ */
+public class StompSessionManagerTests {
+
+	@Test
+	public void testDoConnectFailure() throws Exception {
+		StompClientSupport stompClient = mock(StompClientSupport.class);
+		stompClient.setTaskScheduler(new ConcurrentTaskScheduler());
+		AbstractStompSessionManager sessionManager = new AbstractStompSessionManager(stompClient) {
+
+			private final AtomicBoolean thrown = new AtomicBoolean();
+
+			@Override
+			protected ListenableFuture<StompSession> doConnect(StompSessionHandler handler) {
+				if (!this.thrown.getAndSet(true)) {
+					throw new RuntimeException("intentional");
+				}
+				else {
+					SettableListenableFuture<StompSession> future = new SettableListenableFuture<StompSession>();
+					StompSession stompSession = mock(StompSession.class);
+					future.set(stompSession);
+					handler.afterConnected(stompSession, getConnectHeaders());
+					return future;
+				}
+			}
+
+		};
+
+		sessionManager.start();
+
+		final SettableListenableFuture<StompSession> stompSessionFuture = new SettableListenableFuture<StompSession>();
+		sessionManager.connect(new StompSessionHandlerAdapter() {
+
+			@Override
+			public void afterConnected(StompSession session, StompHeaders connectedHeaders) {
+				stompSessionFuture.set(session);
+			}
+
+		});
+
+		assertNotNull(stompSessionFuture.get(10, TimeUnit.SECONDS));
+
+		sessionManager.stop();
+	}
+
+}