Add an example to show all hostvars using debug

This is a useful example to help debug how facts and vars are being collated.
Update facts.py

Fixes #7093 --devel
template: When rendering exception from Jinja, include exception name.

This will give basic context or *what* the error is, otherwise there're
messages like:

fatal: [tcwg.ci.linaro.org] => {'msg': 'files/private/jenkins_ssh_publishers.xml.tcwg-ci', 'failed': True}

Which give a filename related to error, but not an idea what error is.

Addresses #7103.
template: Handle TemplateNotFound to render more helpful error message.

At the point the exception is handled, it is likely due to error loading
a sub-template included from main template. Besides file not found, it
can be caused also by include path failing Jinja2 checks.