Add more info to error messages

Instead of:

    fatal: [vagrant] => One or more undefined variables: 'start_command' is undefined

we now get:

    fatal: [vagrant] => One or more undefined variables: 'start_command' is undefined
    (module_name: debug; module_args: msg="start_command = {{ start_command }}";
    file: /Users/marca/ansible/roles/pythonapp/tasks/main.yml)

which immediately points me to the error in a large library of roles.

Cc: @sontek, @sudarkoff