Better error detection
better error detection according to changes in server-side-backup JSON response, see https://gitlab.labs.nic.cz/turris/server-side-backups/commit/964733a65370e7508ad40c0e73e8cf24c888a481 and https://gitlab.labs.nic.cz/turris/server-side-backups/commit/bc010f7652555cc1b186122c5bf744df87c8c5d9