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.
2 jobs for work/add-postbuild-msg in 2 minutes and 25 seconds (queued for 78 minutes and 36 seconds)
Status Job ID Name Coverage
  Test
passed #46954
smoke-test

00:01:48

passed #46953
unit-test

00:00:37