ReaR: backup using tar always succeeds, even in case of tar error

Issue

• When backing up using the NETFS and tar option, backup errors are not detected, ReaR continues and displays the following message

  WARNING: tar ended with return code 1 and below output:
    ---snip---
  
    ----------
  This means that files have been modified during the archiving
  process. As a result the backup may not be completely consistent
  or may not be a perfect copy of the system. Relax-and-Recover
  will continue, however it is highly advisable to verify the
  backup in order to be sure to safely recover this system.