Have FailingXdsStream call methods be no-ops rather than throw Unsupp…

…ortedOperationException. Pass a Status to FailingXdsTransport instead of a message and having it generate the Status.

xds/src/main/java/io/grpc/xds/client/ControlPlaneClient.java

@@ -497,10 +497,10 @@ private void cleanUp() {
 
   @VisibleForTesting
   static class FailingXdsTransport implements XdsTransport {
-    String failingMessage;
+    Status error;
 
-    public FailingXdsTransport(String msg) {
-      this.failingMessage = msg;
+    public FailingXdsTransport(Status error) {
+      this.error = error;
     }
 
     @Override
@@ -520,22 +520,22 @@ private class FailingXdsStreamingCall<ReqT, RespT> implements StreamingCall<ReqT
 
       @Override
       public void start(XdsTransportFactory.EventHandler<RespT> eventHandler) {
-        eventHandler.onStatusReceived(Status.UNAVAILABLE.withDescription(failingMessage));
+        eventHandler.onStatusReceived(error);
       }
 
       @Override
       public void sendMessage(ReqT message) {
-        throw new UnsupportedOperationException();
+        // no-op
       }
 
       @Override
       public void startRecvMessage() {
-        throw new UnsupportedOperationException();
+        // no-op
       }
 
       @Override
       public void sendError(Exception e) {
-        throw new UnsupportedOperationException();
+        // no-op
       }
 
       @Override

xds/src/main/java/io/grpc/xds/client/XdsClientImpl.java

@@ -487,7 +487,8 @@ private ControlPlaneClient getOrCreateControlPlaneClient(ServerInfo serverInfo)
       String msg = String.format("Failed to create xds transport for %s: %s",
           serverInfo.target(), e.getMessage());
       logger.log(XdsLogLevel.WARNING, msg);
-      xdsTransport = new ControlPlaneClient.FailingXdsTransport(msg);
+      xdsTransport =
+          new ControlPlaneClient.FailingXdsTransport(Status.UNAVAILABLE.withDescription(msg));
     }
 
     ControlPlaneClient controlPlaneClient = new ControlPlaneClient(