skaffold dev does not log port-forward errors

When using `skaffold dev` with a profile that has `portForwarding`, the port-forward can encounter errors that don't show in the logs.

This is problematic when the file watch functionality of `skaffold dev` rolls out changes to a deployment, which changes the pods and causes the next request to fail.

With `skaffold dev`, the next request will fail with no logs shown in skaffold or in the server.

When the port-forward is established manually via kubectl, you see these errors:

```
E0505 16:21:52.279993 1912197 portforward.go:413] "Unhandled Error" err="an error occurred forwarding 50051 -> 50051: error forwarding port 50051 to pod , uid : network namespace for sandbox \"\" is closed"
error: lost connection to pod
```

Ideally, we would see port-forward errors in the `skaffold dev` logs, so that it is obvious the failure coming from the port-forward, rather than from an issue with your application.