validate: improve message printed in check_devices.yml

validate: improve message printed in check_devices.yml

The message prints the whole content of the registered variable in the
playbook, this is not needed and makes the message pretty unclear and
unreadable.

```
"msg": "{'_ansible_parsed': True, 'changed': False, '_ansible_no_log': False, u'err': u'Error: Could not stat device /dev/sdf - No such file or directory.\\n', 'item': u'/dev/sdf', '_ansible_item_result': True, u'failed': False, '_ansible_item_label': u'/dev/sdf', u'msg': u\"Error while getting device information with parted script: '/sbin/parted -s -m /dev/sdf -- unit 'MiB' print'\", u'rc': 1, u'invocation': {u'module_args': {u'part_start': u'0%', u'part_end': u'100%', u'name': None, u'align': u'optimal', u'number': None, u'label': u'msdos', u'state': u'info', u'part_type': u'primary', u'flags': None, u'device': u'/dev/sdf', u'unit': u'MiB'}}, 'failed_when_result': False, '_ansible_ignore_errors': None, u'out': u''} is not a block special file!"
```

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1719023
(cherry picked from commit e6dc3ebd8c8161e56c0a4a1fdb409272b9fd5342)
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>