(#7276) Better reporting from the plugin download action.

Instead of just returning vague values, return useful information when
rendering for a human being.

Based on work by Luke Kaines <luke@puppetlabs.com> in
https://github.com/lak/puppet/commit/a61cc770ca9b2cad744b5b21b9776a834d6ca895

Reviewed-By: Pieter van de Bruggen <pieter@puppetlabs.com>