add more meaningful error message when the invalid config is caused by undefined maxapplications

    partitions:
      - name: default
        queues:
          - name: root
            parent: true
            submitacl: "*"
            maxapplications: 2
            queues:
              - name: parent 

The above config can produce following error message.

"parent maxApplications must be larger than child maxApplications".

It seems to me the error message does not cover the root cause (maxapplications is zero). 

https://github.com/apache/yunikorn-core/blob/master/pkg/common/configs/configvalidator.go#L207