Give more information when a task is empty

I made a typo in a playbook and was great by:

    ERROR: expecting dict; got: None

The issue was a single - on the last line of a playbook.
With the name of the file, I was able to see right away where the
error was.
Merge pull request #7126 from mscherer/better_error_message_empty_task

Give more information when a task is empty