Backups with velero have issues

## What version of Knative?

0.21.0

## Expected Behavior

Backing up and restoring a Knative service with https://velero.io/ should succeed - or at a minimum have clearer error messages.

## Actual Behavior

Restore fails with confusing messages

```
time="2021-03-09T11:11:38Z" level=info msg="error restoring helloworld-python: admission webhook \"validation.webhook.serving.knative.dev\" denied the request: validation failed: missing field(s): metadata.labels.serving.knative.dev/service"
```

In fact the labels are present but since they don't match the owner references (those are stripped by velero). The error message should be more clear. Another instance error is getting the `X does not own Y` error.

We can fix the label error message pretty easily. But should figure out if we're doing things that break _other_ backup tools and decide if there's a mitigation to be had

## Steps to Reproduce the Problem