Apologies if I had a little more time, I would have had a go at submitting an actual change.
Whilst investingating some other problem, I accidentally uninstalled mariabackup without realising. My wsrep join began failing. however, it is failing without the intended error.
Having looked at how other wsrep providers handle this, I see there is a method in `wsrep_sst_common` called `wsrep_check_programs`. I wasn't sure of this was an oversight, or if there was a specific reason mariabackup is checked using an alternative mechanism.
In contrast, this is what wsrep_sst_rsync uses