Improve JettySockJsIntegrationTests to show real error

In a recent CI build failure a test timed out waiting for a message:
https://build.spring.io/browse/SPR-PUB-1316

In fact there was a WebSocket transport failure originating in Jetty's
WebSocket Parser. However this failure is only apparent by looking at
the logs. This change adds a check if a transport error ocurred while
we were waiting and throws an AssertionError.

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/client/AbstractSockJsIntegrationTests.java

@@ -64,6 +64,7 @@ import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.fail;
 
 /**
  * Integration tests using the
@@ -296,6 +297,8 @@ public abstract class AbstractSockJsIntegrationTests {
 
 		private volatile WebSocketSession session;
 
+		private volatile Throwable transportError;
+
 		private volatile CloseStatus closeStatus;
 
 
@@ -309,6 +312,11 @@ public abstract class AbstractSockJsIntegrationTests {
 			this.receivedMessages.add(message);
 		}
 
+		@Override
+		public void handleTransportError(WebSocketSession session, Throwable exception) throws Exception {
+			this.transportError = exception;
+		}
+
 		@Override
 		public void afterConnectionClosed(WebSocketSession session, CloseStatus status) throws Exception {
 			this.closeStatus = status;
@@ -321,12 +329,22 @@ public abstract class AbstractSockJsIntegrationTests {
 
 		public void awaitMessage(TextMessage expected, long timeToWait) throws InterruptedException {
 			TextMessage actual = this.receivedMessages.poll(timeToWait, TimeUnit.MILLISECONDS);
-			assertNotNull("Timed out waiting for [" + expected + "]", actual);
-			assertEquals(expected, actual);
+			if (actual != null) {
+				assertEquals(expected, actual);
+			}
+			else if (this.transportError != null) {
+				throw new AssertionError("Transport error", this.transportError);
+			}
+			else {
+				fail("Timed out waiting for [" + expected + "]");
+			}
 		}
 
 		public CloseStatus awaitCloseStatus(long timeToWait) throws InterruptedException {
-			awaitEvent(() -> this.closeStatus != null, timeToWait, " CloseStatus");
+			awaitEvent(() -> this.closeStatus != null || this.transportError != null, timeToWait, " CloseStatus");
+			if (this.transportError != null) {
+				throw new AssertionError("Transport error", this.transportError);
+			}
 			return this.closeStatus;
 		}
 	}