dev should not error out on introduced error to skaffold.yaml

Follow-on to #4158: skaffold dev shouldn't error out on a typo to `skaffold.yaml`. I made a mistake while uncommenting and forgot to uncomment a line to produce an invalid yaml:
```
Pruning images...
WARN[0217] builder cleanup: pruning images: could not remove image after 5 retries
parsing skaffold config: unable to parse config: yaml: unmarshal errors:
line 7: cannot unmarshal !!seq into v2beta3.KubectlDeploy
```

and Skaffold then exited.