(#14296) Improve template function error message.

The current template function does not provide sufficient detail for
troubleshooting. This patch uses the backtrace to provide the fully
qualified filepath and line number.

Previous error message:
Failed to parse template test/example.erb: undefined method `[]' for
nil:NilClass at /tmp/module/test/manifests/init.pp:2 on node example

New error message:
Failed to parse template test/example.erb:

Filepath: /tmp/module/test/templates/example.erb
Line: 10
Detail: undefined method `[]' for nil:NilClass

at /tmp/module/test/manifests/init.pp:2 on node example