copy thread)

mariabackup hangs waiting until innodb redo log thread read log till certain
LSN, and it waits under FTWRL. If there is redo log read error in the thread,
it is finished, and main thread knows nothing about it, what leads to hanging.
As it hangs under FTWRL, slave threads on server side can be blocked due
to MDL lock conflict.

The fix is to finish mariabackup with error message on innodb redo log read
failure.