Without this patch the error message for this example:

```
---
apiVersion: v1
kind: Pod
metadata:
  name: test
spec:
  containers:
    - name: agent
      image: debian:latest
  tolerations:
    - key: pool
      operator: Exists
      value: build
      effect: NoSchedule
```

Looks like:

```
The Pod "test" is invalid: spec.tolerations[0].operator: Invalid value:
core.Toleration{Key:"pool", Operator:"Exists", Value:"build",
Effect:"NoSchedule", TolerationSeconds:(*int64)(nil)}: value must be
empty when `operator` is 'Exists'
```

To clarify that the `Value` field is wrong, we now directly point the
`field.Invalid` to it. Now the error message becomes a more clear and
concise one:

```
The Pod "test" is invalid: spec.tolerations[0].operator: Invalid value:
"build": value must be empty when `operator` is 'Exists'
```

Signed-off-by: Sascha Grunert <sgrunert@redhat.com>