chore: improve error message of cdk.out is missing (#1055)

Encountered someone at a workshop who had copy/pasted into the wrong file,
replacing their app instantiation and invocation, and the error message was

"could not find cdk.out"

Which was super unhelpful and took me a good while to figure out what had happened.

Pinpoint at least one possible cause of this happening in the error message.

packages/aws-cdk/bin/cdk.ts

@@ -465,7 +465,7 @@ async function initCommandLine() {
         const outfile = await exec();
         debug('outfile:', outfile);
         if (!(await fs.pathExists(outfile))) {
-          throw new Error(`Unable to find output file ${outfile}`);
+          throw new Error(`Unable to find output file ${outfile}; are you calling app.run()?`);
         }
 
         const response = await fs.readJson(outfile);