assert: add optional `msg` argument

Sample playbook when it may be useful:
```
- uri: return_content=yes url={{ api_base }}/action/{{ inventory_hostname }}
  register: output
- assert:
    msg: '{{ output.json | to_nice_json }}'
    that:
      - output.json.foo != "bar"
      - output.json.boo == 42
```