Improve error message when multiple action are given

This can be illustrated using this playbook:

    - command: id
      delegate_to: "{{ remote_server }}"
      user: "{{ remote_user }}"

The error is to use 'user' instead of 'remote_user', but the error message
do not really mention it, so it can be a bit hard to spot.
Merge pull request #7083 from mscherer/better_message_multiple_action

Improve error message when multiple action are given