git: Show warning at end of build if git-stash fails.

Authored by mpyne on May 18 2020, 2:12 AM.

Description

git: Show warning at end of build if git-stash fails.

If this all works this would allow us to migrate this functionality to
improve how we handle git-stash as in issue #42.

In my testing, it works at least. I'll show example output in the
corresponding merge request for tracking.