Low: galera: report when check_user is unable to retrieve status

This is an important piece of data as it reflects that the check_user
does not have permissions to view status. Otherwise this error would
appear to signify that the galera instance did not start properly in
Primary mode, which could result in the user investigating the wrong
problem.
Merge pull request #462 from davidvossel/galera-agent

Low: galera: report when check_user is unable to retrieve status