load tasks/vars includes in syntax-check mode as well



Hi,

it appears that the ansible-playbook --check-syntax site.yml does not check includes under the "tasks:" or "vars:" sections.

E.g.
```
$ cat site.yml
- hosts: all
  vars_files:
  - vars/no-file-here

$ ansible-playbook --syntax-check site.yml 
Playbook Syntax is fine
```
However:
```
$ ansible-playbook --list-tasks site.yml  
playbook: site.yml
ERROR: file not found: /tmp/vars/not-there
```
This branch will fix this and will error in the same way as --list-tasks when --syntax-check is used.

Thanks for your consideration,
Michael