Return error if madatory field is missing

[PrasadG193]

Currently, kyaml2go does not return an error if a mandatory fields is missing. (e.g container.name or container.image in deployment)
e.g

apiVersion: apps/v1
kind: Deployment
metadata:
  name: test-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: demo
  template:
    metadata:
      labels:
        app: demo
    spec:
      containers:
      - imagePullPolicy: IfNotPresent
        ports:
        - containerPort: 80
          name: http
          protocol: TCP