App gets stuck deleting if service account is not created

**What steps did you take:**
Installed the "simple-app" from the instructions without first creating the service account for it.
https://github.com/vmware-tanzu/carvel-kapp-controller/blob/develop/docs/walkthrough.md

Tried to delete the app via `kapp -a simple-app delete` and see it hang/timeout.

**What happened:**
```
$ k get app
NAME DESCRIPTION SINCE-DEPLOY AGE
simple-app Delete failed: Preparing kapp: Getting service account: serviceaccounts "default-ns-sa" not found 3s 56m
```

**What did you expect:**
Delete should succeed

**Anything else you would like to add:**

**Environment:**

- kapp Controller version : v0.16.0
- Kubernetes version:
```
Client Version: version.Info{Major:"1", Minor:"19", GitVersion:"v1.19.3", GitCommit:"1e11e4a2108024935ecfcb2912226cedeafd99df", GitTreeState:"clean", BuildDate:"2020-10-14T12:50:19Z", GoVersion:"go1.15.2", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"20", GitVersion:"v1.20.2", GitCommit:"faecb196815e248d3ecfb03c680a4507229c2a56", GitTreeState:"clean", BuildDate:"2021-01-21T01:11:42Z", GoVersion:"go1.15.5", Compiler:"gc", Platform:"linux/amd64"}
```