Errors ignore output string type

If a configuration is missing 'apiversion', 'type', etc, keys, kubeval dumps the issue to log.Error, prints null, and exits.
When the -o flag is specified as json, I would expect kueval to report the error in json format.

Example
```
# test.yaml

apiVersion: apps/v1
#kind: Deployment
metadata:
name: sandbox
spec:
replicas: astring
selector:
matchLabels:
app: sandbox
template:
metadata:
labels:
app: moda-sandbox
spec:
dnsPolicy: Default
containers:
- name: sandbox
image: sandbox
ports:
- name: http
containerPort: 8080
protocol: TCP
envFrom:
- configMapRef:
name: kube-cluster-metadata
```

Current output:
```
$ ./bin/kubeval -d test.yaml -o json
ERR - test.yaml: Missing 'kind' key
null
```

Possible JSON output:
```
$ ./bin/kubeval -d test.yaml -o json
[
{
"filename": "test.yaml",
"kind": "Deployment",
"status": "invalid",
"errors": [
"spec.replicas: Missing 'kind' key"
]
}
]
```