Currently MariaBackup's stderr contains records in different formats, in addition to server-style messages.

Some lines come with a timestamp and apparently a thread ID or something, some with a timestamp without a thread ID, some with thread ID without a timestamp, and some without any of those:

[01] mariabackup: Warning: We assume the table was dropped during xtrabackup execution and ignore the file.

[01] mariabackup: Warning: skipping tablespace test/seq5.

190110 22:12:01 [01] Copying ./test/t6.ibd to /home/travis/logs/vardir/backup_0/test/t6.ibd

...

DDL tracking :  create 253 "./test/#sql-6054_10.ibd": 1a10018

190110 22:12:01 Acquiring BACKUP LOCKS...

It would be good (and more informative) if all records contained a timestamp at least. mariabackup prefix might also be good, to distinguish the records from server-side ones when they are intermixed.

Server-side ones also vary, but that's out of the scope of this task. Here is a decent one, for example:

2019-01-10 22:12:00 0 [Note] InnoDB: Number of pools: 1