When there is error in taking backup of a table from bad tablespace, XtraBackup didn't produce error message at the end but reported "COMPLETED OK" and also RETURN CODE (echo $?) was 0. Proceeded with prepare and confirmed that the tablespace is lost and found only .frm under datadir for the reported table. We need to add Error message with the error RETURN CODE and so the DBA/automated jobs could identify the errors with their backup.
Vinodhs-MBP:email@example.com vinodhkrish$ xtrabackup --backup -uroot --target-dir=/usr/local/mysqlbackup/databackup_7 > backup_7.log 2>&1
Vinodhs-MBP:firstname.lastname@example.org vinodhkrish$ echo $?
Vinodhs-MBP:email@example.com vinodhkrish$ cat backup_7.log
xtrabackup: Generating a list of tablespaces
InnoDB: Allocated tablespace ID 2 for mysql/innodb_index_stats, old maximum was 0
InnoDB: Space ID in fsp header is 117440512, but in the page header it is 2231369728.
InnoDB: A bad Space ID was found in datafile: ./vinodh/testbackup.ibd, Space ID:18446744073709551615, Flags: 1073741824. Please refer to http://dev.mysql.com/doc/refman/5.7/en/innodb-troubleshooting-datadict.html for how to resolve the issue.
180206 18:16:47  Copying ./ibdata1 to /usr/local/mysqlbackup/databackup_7/ibdata1
180206 18:16:47  ...done
180206 18:16:51 completed OK!