MDEV-30822: Adjust wsrep_sst_mariabackup.sh to understand both pre-MDEV-18931 and post-MDEV-18931 file naming

This commit adds support for legacy names for files such
as mariadb_backup_galera_info, mariadb_backup_checkpoints
and mariabackup_binlog_info to allow upgrading from old
to new server versions without stopping the galera cluster.

In addition, improved handling of errors to prevent hangs
detected during stress testing of the updated SST script
for mariabackup (when conditions for hangs at the mariabackup
stage were artificially created).