mariabackup does not complain on bad backup-dir

When a bad --backup-dir location is choosen mariabackup does not complain but just falls back to default $HOME/xtrabackup_backupfiles

This is IMHO a bad behaviour for Enterprise Class systems and against the fail-fast approach.

shell> mariabackup --user=root --backup-dir=/tmp/backup/full --backup
Using server version 10.1.23-MariaDB
mariabackup based on MariaDB server 10.1.23-MariaDB Linux (x86_64)
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /home/mysql/data/mariadb-101/
xtrabackup: open files limit requested 0, set to 16384
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = ./
xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 16777216
170516 14:47:04 >> log scanned up to (5297301595)
xtrabackup: Generating a list of tablespaces
170516 14:47:04 [01] Copying ./ibdata1 to /home/mysql/xtrabackup_backupfiles/ibdata1
...
170516 14:48:40 Backup created in directory '/home/mysql/xtrabackup_backupfiles/'
MySQL binlog position: filename 'laptop4_mariadb-10.000047', position '5908951', GTID of the last change '0-3306-3180130'
170516 14:48:40 [00] Writing backup-my.cnf
170516 14:48:40 [00] ...done
170516 14:48:40 [00] Writing xtrabackup_info
170516 14:48:40 [00] ...done
xtrabackup: Transaction log of lsn (5292811958) to (5300480729) was copied.
170516 14:48:42 completed OK!