Output of `render` is corrupted with error messages

### Expected behavior

If something goes wrong during `skaffold render`, it should print a message to stderr/stdout and exit with a non-zero status code if the problem is fatal.

### Actual behavior

Error messages are included in the actual output file and `render` returns a zero exit code.

### Information

- Skaffold version: v1.25.0
- Operating system: Linux
- Installed via: skaffold.dev

The error in this case looks like this:

```
kubectl client version: unknown
couldn't get kubectl minor version: strconv.Atoi: parsing "unknown": invalid syntax
```

See cl/377891886 internally.

Skaffold is being run as `skaffold render --digest-source=local --loud --output=release.yaml`.

Standard output includes these warnings:

```
time="2021-06-07T12:19:02Z" level=warning msg="unable to get kubectl client version: starting command kubectl version --client -ojson: exec: \"kubectl\": executable file not found in $PATH"
time="2021-06-07T12:19:02Z" level=warning msg="k8s/*.yaml did not match any file"
time="2021-06-07T12:19:02Z" level=warning msg="unable to get kubectl client version: starting command kubectl version --client -ojson: exec: \"kubectl\": executable file not found in $PATH"
```

So the underlying issue here is probably that kubectl isn't installed in my build environment. But the rendered manifest is otherwise fine, so ideally this should just be a warning that doesn't mess up the output. At worse this should cause skaffold to fail with an error so that the output isn't silently corrupted.