chore: throw more descriptive error when failing to create output dir…

…ectory (aws#18296)

I passed a `--output` to the CDK that was `/public/something/or/other`, on a system with no `/public`, and then forgot about it. I was presented with the head-scratch-inducing failure message:

```
ENOENT: no such file or directory, mkdir '/public'
Error: ENOENT: no such file or directory, mkdir '/public'
```

This should improve the error so that some reference is made to why the CDK expects or desires this path to exist (and remind me that in fact I told it to go there).

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/lib/api/cxapp/exec.ts

@@ -68,7 +68,11 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
   if (!outdir) {
     throw new Error('unexpected: --output is required');
   }
-  await fs.mkdirp(outdir);
+  try {
+    await fs.mkdirp(outdir);
+  } catch (error) {
+    throw new Error(`Could not create output directory ${outdir} (${error.message})`);
+  }
 
   debug('outdir:', outdir);
   env[cxapi.OUTDIR_ENV] = outdir;