Adding warn message for missing route to fix first part of #978 (#979)

Co-authored-by: Thomas Gray <tomgray@amazon.co.uk>

Author: iamtgray

src/deployments/cdk/src/deployments/transit-gateway/step-3.ts

@@ -235,6 +235,10 @@ function CreateRoute(props: {
     }
   } else {
     const routeId = routesMap[routeName];
+    if (!routeId) {
+      console.warn(`Cannot find route ${routeName} in transit gateway ${transitGateway.name}`);
+      return;
+    }
     CreateTransitGatewayRoute({
       scope,
       name: routeName,