Description of problem:
I was using custom_compatibility_version parameter of ovirt_vm module to set custom compatibility version of a VM to value higher than engine's version. I had engine of version 4.3 and set custom compatibility version to 4.4. This of course shouldn't work and that's all right. However, if use attempts it, they should get back some sensible error message from Ansible. Meanwhile, I got this:

fatal: [localhost]: FAILED! => {"changed": false, "msg": "Fault reason is \"Operation Failed\". Fault detail is \"java.lang.NullPointerException\". HTTP response code is 500."}


Version-Release number of selected component (if applicable):
ansible 2.8.1
rhvm-4.3.5.1-0.1.el7.noarch

How reproducible:
100 %

Steps to Reproduce:
1. Have a VM in your 4.3 engine (test_vm in our case)
2. Run this playbook:

- name: Try setting too high custom compatibility version
  hosts: localhost
  tasks:
    
    - name: Obtain SSO token with using username/password credentials
      ovirt_auth:
        url: https://<your_engine>/ovirt-engine/api
        username: admin@internal
        password: '<your_pass>'
        insecure: yes 

    - name: Set custom compatibility version
      ovirt_vm:
        auth: "{{ ovirt_auth }}"
        name: test_vm
        custom_compatibility_version: 4.4 

Actual results:
java.lang.NullPointerException is returned

Expected results:
Some informative, user-friendly message should be returned